The U.S. accused Russia of escalating the Syrian conflict by sending attack helicopters to President Bashar Assad's regime, and UN observers were attacked Tuesday with stones, metal rods and gunfire that blocked them from a besieged, rebel-held town where civilians were feared trapped by government shelling.

There was no immediate reaction from the Russian Foreign Ministry on the accusation by U.S. Secretary of State Hillary Rodham Clinton.

Moscow insists that any arms it supplies to its Damascus ally are not being used against anti-government protesters in the 15-month-old uprising.

"We are concerned about the latest information we have that there are attack helicopters on the way from Russia to Syria, which will escalate the conflict quite dramatically," Clinton said in Washington.

With diplomacy at a standstill, the reported shipment of helicopters suggests a dangerous new turn for Syria after more than a year of harsh government crackdowns on mainly peaceful protests and the emergence of an increasingly organized armed insurgency.

UN Under-Secretary General for Peacekeeping Operations Herve Ladsous was asked in an interview Tuesday with Reuters if the Syrian conflict could be characterized as a civil war, and he responded affirmatively.

"Yes, I think we can say that. Clearly what is happening is that the government of Syria lost some large chunks of territory in several cities to the opposition and wants to retake control of these areas," he told Reuters.

The news agency says it's the first time a senior UN official has described that the Syrian crisis as a civil war.

UN observers have seen a steep rise in violence and a dangerous shift in tactics by both sides in Syria in the last five days, UN spokesman Kieran Dwyer said.

The Syrian government, intent on wresting back control of rebel-held areas, is shelling heavily populated districts and using attack helicopters over cities "with devastating impact on civilians," Dwyer said. The opposition, in turn, is increasingly co-ordinating attacks against government forces and civilian infrastructure, and "the conflict has reached all parts of Syria virtually," he said.

Observers met by angry crowd

The UN observers attacked Tuesday were not hurt as they were turned back by the assault on their vehicles by an angry crowd near the town of Haffa, the UN said. The source of the gunfire was not clear.

'The longer this violence continues, the more dangerous it becomes not only for the country and the Syrian people but the region.'—UN spokesman

Activists blamed regime loyalists for the attack. The violence raised questions about the ability of about 300 unarmed monitors to provide a useful assessment of a country that is spiralling toward civil war.

"All UN observers are now back at their bases and are secure," said Sausan Ghosheh, a spokeswoman for UN observers in Syria. She said monitors have been trying to reach Haffa since June 7.

During the conflict, the observers have been prevented from entering other flash point areas. Last week, it took days for the monitors to reach Mazraat al-Qubair, where nearly 80 people were reported slain, because government troops and residents blocked them.

Murderous grind

As the conflict deteriorates into a murderous grind, regional power brokers from Iran to Turkey risk getting drawn into the fight.

Diplomatic hopes have rested on Russia — Syria's most important ally and protector — agreeing on a transition plan that would end the Assad family dynasty, which has ruled Syria for more than four decades.

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A resident, left, shows a shell to a United Nations observer, right, after shelling in the Talbisah area of the city of Homs, Syria, Monday. (David Manyua/UN/Associated Press)

But Moscow has rejected outside forces to end the conflict or any plan to force regime change in Damascus. Despite withering criticism from the West, it insists that any arms it supplies to Syria are not being used against civilians.

The UN's special envoy on Syria, Kofi Annan, asked governments with influence to "twist arms" to end the bloodshed. But there are few signs the diplomatic pressure is having any measurable effect.

"The longer this violence continues, the more dangerous it becomes not only for the country and the Syrian people but the region," Annan's spokesman, Ahmad Fawzi, told reporters in Geneva.

"It's dangerous and the red light is flashing."

The deteriorating situation in Haffa has raised alarm in the past eight days, and Washington said Monday that regime forces may be preparing a massacre in the village, which is about 30 kilometres from Assad's hometown of Kardaha in Latakia province along the Mediterranean coast.

Activists said government forces were firing mortar rounds into the village.

Journalists restricted

Calls to the area did not go through Tuesday. The government restricts journalists from moving freely, making it nearly impossible to independently verify accounts from either side.

Violence has escalated significantly in recent days as the government fights to reassert control of pockets of resistance across the country. Syrian forces fired mortars at protesters Tuesday in the eastern city of Deir el-Zour, killing at least 10 people, activists said. Clashes also were reported in central Homs province.

Amateur video of the mortar attack on Deir el-Zour showed some of the dead in a street as survivors screamed in panic and tried to remove their bodies. Other videos showed some of the wounded being treated at a hospital.

The state news agency SANA blamed the Haffa violence on terrorists — the term it uses to describe rebels — who had attacked residents in the village.

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A shell is seen on a street in a residential area of Talbisah in Homs city Syria on Monday. (David Manyua/UN/Associated Press)

Throughout the uprising, violence has rattled Latakia, an area of profound importance to the regime. Latakia province is the heartland of the Alawite minority to which Assad and the ruling elite belong, although there is a mix of religious groups.

Syria's Sunni majority makes up the backbone of the opposition, and minorities such as Alawites and Christians have generally stuck to the sidelines, in part out of fears that they will be marginalized — or even face retribution — if Sunnis take over. As the violence worsens, world powers have struggled to find ways to end the bloodshed. Annan brokered a peace plan that was supposed to go into effect April 12 but never took hold. More than 13,000 people have died in Assad's crackdown on the uprising that began in March 2011 as part of the Arab Spring, according to activist groups.

Annan renewed calls Tuesday for the bloodshed to end.

"It is totally unacceptable and it must stop," spokesman Fawzi said, "and that is why Annan has invited governments with influence to raise the bar to another level, to the highest level possible, and twist arms if necessary, to get the parties to implement the plan."

He didn't specify the countries that might still have leverage with the Assad regime, but Russia, China and Iran are Syria's closest and strongest allies.

Fawzi said it was up to the government to take the first step to end the violence.

"The stronger party should send a strong signal in good faith and stop the violence, and the stronger party in this case is clearly the government of Syria," he said.

Syria has so far appeared largely impervious to the chorus of condemnation that began shortly after government forces launched a crackdown on peaceful protests. Economic sanctions have had little effect.

Russia and China have used their veto power at the UN Security Council to block strong action against Damascus to prevent any international military intervention against Syria.

But the U.S. and its allies also have shown little appetite for getting involved in another Arab nation in turmoil.

British Foreign Secretary William Hague said Britain and other Western powers were not seeking a foreign military intervention, despite his warning Monday that all options must remain open if diplomacy doesn't work.

'Syria not another Libya'

"Clearly, we are not looking for any foreign military intervention, and we should not think about it in terms of another Libya," Hague said during a two-day visit to Pakistan.

"All our efforts are going into supporting a peaceful transition in Syria and a peaceful solution, because any violent solution would clearly involve many more deaths and a great deal more hardship for the Syrian people," he said.

Besides the potential for more violence, there also is a real concern it could spill over to other countries in the region.

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Young demonstrators protest against Syria's President Bashar Assad in northwestern Syria on Sunday. A UN report alleges regime forces have tortured and killed children during the ongoing conflict with opposition groups. (Raad Al Fares/Shaam News Network/Reuters)

The International Organization for Migration said more than 5,000 Syrian refugees have now crossed into northern Iraq, and their numbers are growing.

"With the help of international agencies we have managed to provide them with basic necessities, including shelter, food and water since the camp opened at the beginning of April," Shakir Yasseen Yasseen, director general of the Regional Government's Bureau of Migration and Development. "But our prognosis is that they will not be going back home anytime soon."

Tuesday's attack on the monitors comes a day after the release of a UN report that includes Syrian government forces and their allied shabiha militias for the first time on a list of 52 governments and armed groups that recruit, kill or sexually attack children in armed conflicts around the world.

Children as young as nine years old have been victims of killing and maiming, arbitrary arrest, detention, torture and sexual violence, and have even been used as human shields, the report said.

UN report details violations against children

They include:

  • Somalia: 7,799 child casualties of conflict last year, just in the three main hospitals in the capital Mogadishu. Recruitment of 948 youngsters by the Islamic militant group al-Shabaab, and 242 cases of rape and sexual violence.

  • Afghanistan: 1,325 children killed or injured in 2011, 30 per cent by improvised explosive devices placed by armed groups.

  • Iraq: 146 children reportedly killed and 265 injured as a result of violence last year.

  • Libya: Since the uprising began in February 2011, the presence of children in armed forces and armed groups "was broadly reported."

  • Ivory Coast: 271 cases of sexual violence, including six against boys.

"In almost all recorded cases, children were among the victims of military operations by government forces, including the Syrian armed forces, the intelligence forces and the shabiha militia, in their ongoing conflict with the opposition, including the Free Syrian Army," the report said.

Dwyer told CBC that such abuse of children in Syria "is a deeply serious and troubling problem. Things have only gotten worse since that report was released."

In the latest violence, on Tuesday, Syrian forces pelted the eastern city of Deir el-Zour with mortars as anti-government protesters were dispersing before dawn Tuesday, killing at least 10 people.

Amateur video of the mortar attack showed some of the dead in a street as survivors screamed in panic and tried to remove their bodies. Other videos showed some of the wounded receiving treatment at a hospital. The Local Co-ordination Committee and the Britain-based Syrian Observatory for Human Rights said 10 people died in the shelling.

Following Monday's release of the UN report, Secretary General Ban Ki-moon's said the United Nations has received reports of "grave violations" against children in Syria since March 2011, when protests against Assad's government began.

With files from CBC News