Tensions between the United States and Russia flared Wednesday as the former Cold War foes traded blame for the violence in Syria just days before a planned meeting between U.S. President Barack Obama and Russian President Vladimir Putin.

Secretary of State Hillary Rodham Clinton held to her explosive accusation that the "latest information" in U.S. hands is that Russia is sending attack helicopters to Syrian President Bashar Assad's regime at the risk of fomenting a dangerous civil war.

Russian Foreign Minister Sergey Lavrov fired back by alleging that the U.S. has sent military support to the region with the same result.

The public U.S.-Russian rift is occurring at a time the Obama administration had hoped to court Moscow's support for a transition plan to end the Assad regime. If nothing else, the dispute underlines the American government's continued difficulty in finding a strategy to pacify Syria after 15 months of brutal government crackdowns and armed rebellion.

A day after blasting Moscow for purportedly sending new helicopter gunships to Syria, Clinton lamented on Wednesday that repeated U.S. requests to the Russian government to suspend its military ties with Damascus had fallen on deaf ears.

"We have repeatedly urged the Russian government to cut these military ties completely and to suspend all further support and deliveries," Clinton told reporters. "We know, because they confirm, that they continue to deliver and we believe that the situation is spiralling toward civil war. It is now time for everyone in the international community, including Russia ... to speak to Assad in unified voice and insist that the violence stop."

Clinton questioned Russia's insistence that "it wants peace and stability restored" and that it is not wedded to Assad's remaining in power.

In Tehran, Lavrov rejected the helicopter charge and blamed Washington for fuelling the conflict. He said his government was completing earlier weapons contracts with Syria exclusively for air defence systems, which generally refers to surface-to-air missiles, radar and other such materiel. He didn't speak specifically about helicopters but insisted that nothing being delivered could be used against peaceful demonstrators.

Lavrov was widely quoted as accusing the U.S. of providing Syrian dissidents with weapons, but he only said the U.S. was supplying "special means" to the region, not the rebels.

"We are not supplying to Syria or anywhere else things that are used in fighting with peaceful demonstrators, in contrast to the United States, which is regularly sending such special means to countries in the region," he said. "For some reason, the Americans consider this to be in order. We are not delivering such means and are delivering only that which Syria requires in the event of an armed attack on it from outside."

Clinton denied that charge Wednesday: "The United States has provided no military support to the opposition. None."

Responding to Lavrov's dismissal of the U.S. helicopter claims, U.S. State Department spokeswoman Victoria Nuland said: "I would encourage him to check with his own authorities."

"Russian and Soviet-made helicopters form the base of the Syrian helicopter fleet," Nuland told reporters. "We are seeing these helicopters used all over Syria now against civilians."

In making the initial charge on Tuesday, Clinton cited what she called the "latest information" the U.S. had about helicopters on the way from Russia to Syria. The remark appeared to catch many in the Obama administration unprepared, but two U.S. officials said that Clinton was repeating information contained in a classified intelligence briefing circulated Tuesday morning.

At the White House, press secretary Jay Carney softened Clinton's accusation against Russia, calling it one element of a larger argument the U.S. is making that Russia should do more to spur political change in Syria. He would not say whether Obama and Putin would discuss arms sales on the sidelines of the meeting of the Group of 20 industrial and emerging market nations in Mexico next week.

"Our argument has been, to the Russians and others who have supported that regime in the past, that that it is the wrong thing to do to continue that support," Carney said.

Sanctions plan repeatedly blocked

Diplomatic hopes rest with Washington and Moscow agreeing on a transition plan that might end the four-decade Assad regime. Russia, along with China, has twice blocked the UN Security Council from setting world sanctions on Assad's regime, and Moscow has consistently rejected the use of outside forces to end the conflict or any international plan to force regime change in Damascus.

More than 13,000 people have died since March 2011, according to opposition groups, and the view of many in the international community is that the conflict could get worse still. Hoping for a plan that wins international unity and avoids the need for another U.S. military intervention in the Muslim world, the Obama administration has been trying to get Russia to join a widened diplomatic strategy for a structured end to the four-decade Assad dynasty.

One concession to Moscow is that Assad would be allowed to remain in power for the start of the transition. But Russia has up to now steadfastly backed its closest Middle East partner. Moscow and Damascus maintain long-standing military relations and the Arab country hosts Russia's only naval base in the Mediterranean Sea.

UN mediator Kofi Annan is also banking on a Russian-American understanding on Syria, inviting both powers to a conference aimed at mapping out a transition planned for later this month in Geneva.

Carney expressed the administration's frustration.

"The window of opportunity to bring about a transition to a democratic future for Syria is closing and will close," he said.

"And if it does, the chance for a broader and sectarian civil war will be enhanced greatly."

Is it or isn't it a civil war?

Also Wednesday, Syria denied it is in a state of civil war, saying the country is facing "an armed conflict to uproot terrorism."

The regime statement came on the same day France echoed a statement the day before by UN peacekeeping chief Herve Ladsous saying Syria is indeed in a civil war.

French Foreign Minister Laurent Fabius told a news conference in Paris: "When many groups belonging to the same people tear each other apart and kill each other, if you can't call it a civil war, then there are no words to describe it."

He added that to stop "this civil war from worsening," Assad must leave power and Syrian opposition groups must start a new government. He said he will be in personal contact with the opposition inside Syria.

The Syrian Foreign Ministry said earlier that Ladsous's statements lacked objectivity, and were inaccurate and far from reality.

The ministry statement said authorities in Syria are confronting armed groups engaged in "killings, kidnappings and other terrorist acts." Syrian authorities often refer to rebels fighting the regime in Syria as terrorists.

In other news Wednesday, the International Committee of the Red Cross reported three aid workers suffered minor injuries in Syria when an explosion hit their convoy.

The group said the two Syrian Arab Red Crescent volunteers and one ICRC staff member were travelling with other aid workers from Aleppo to Idlib when the blast hit their marked vehicles Wednesday. A spokesman for the ICRC in Geneva said the aid workers were taken to a medical facility and that their injuries are minor.

Hicham Hassan told The Associated Press that it is the first time Red Cross staffers have been injured since the start of violence in Syria last year, and the ICRC doesn't know if it was targeted in the explosion or who was responsible for it.

Also Wednesday, Syrian state TV said regime forces have retaken control of a rebellious mountainous village following eight days of fierce shelling and clashes.

The report said authorities have managed to restore security and order to the region of Haffa and have "cleansed" it from the "armed terrorist groups."

U.S. accused of 'blatant interference'

The rebels had pulled out of Haffa overnight after intense fighting there and in nearby villages in the coastal mountainous province of Latakia.

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In this citizen journalism image made and provided by the Kfar Suseh Coordinating of the Syrian Revolution on Tuesday, anti-Syrian regime protesters raise up their hands and wave revolutionary flags during a demonstration at Kfar Suseh area, in Damascus. (The Kfar Suseh Co-ordinating of the Syrian Revolution/Associated Press)

Haffa is one of several areas where government forces are battling rebels for control. It is particularly important because the town is about 30 kilometres from Assad's hometown of Kardaha in Latakia province.

The state TV report came as Syria lashed out at the United States after Washington warned of possible mass killings in Haffa.

Syria's Foreign Ministry said the remarks by a State Department spokeswoman warning that Assad's forces could commit massacres in Haffa coincided with stepped-up attacks by rebels in the area. The ministry's statement was reported Wednesday by the state-run news agency, SANA.

"The U.S. administration is continuing its blatant interference in the internal affairs of Syria, its open support for the terrorists, covering up the terrorists' crimes," the statement said.

Latakia 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.

Heavy shelling also continued Wednesday in the rebel-held suburbs of Khaldiyeh in the central city of Homs overnight, Abdul-Rahman said. Despite a week's intense shelling, however, Syrian rebels are still clinging to the area. Footage posted by activists from there showed a city covered in a plume of heavy grey smoke. Intermittent thud of shells can be heard, followed by explosions as they slam into buildings.

Rebels, troops exchange fire

In the nearby town of Deir Baalbah, rebels and Syrian troops exchanged fire in residential areas, with rapid snaps of sustained gunfire echoing through the area, according to amateur video said to be from the scene.

Canadian Foreign Affairs Minister John Baird, speaking on his way into Conservative caucus on Parliament Hill in Ottawa on Wednesday morning, condemned the latest assaults, saying: "Obviously the escalation in violence causes us a huge, huge concern.

"We deplore the action and obviously want to see strong, strong measures taken at the UN Security Council, strong and effective UN Security Council sanctions against the Assad regime. The time has come and we’ll obviously be stepping up our diplomatic efforts to push for that."

It was not immediately clear if UN observers in Syria would be able to reach Haffa. On Tuesday, an angry crowd hurled rocks and sticks at the observers' vehicles as they approached the area, forcing them to turn back. The observers were not hurt. Sausan Ghosheh, a spokeswoman for UN observers in Syria, said the monitors have been trying to reach Haffa since June 7.

Meanwhile, Turkey said Wednesday it was concerned that the conflict in Syria could spill over its borders as the number of Syrian refugees in Turkey increased to more than 29,000.

"We are disturbed by the possibility that it could spread to us," Deputy Foreign Minister Naci Koru told state-run TRT television. Koru said 1,400 more Syrian refugees arrived in the past two days in Turkey, increasing the total number to more than 29,000.

With files from CBC News