Government forces shelled the central Syrian city of Homs on Monday, striking a makeshift medical clinic and residential areas and killing at least 17 people in the third day of a new assault on the epicentre of the country's uprising, activists said.

The government denied shelling the city, however, and said "armed terrorist groups" were attacking civilians and police in several neighbourhoods. The state-run news agency also said Monday that gunmen killed three soldiers and captured others at a checkpoint in the Jabal al-Zawiyah region of Idlib province, which borders Turkey.

Syria has blocked access to trouble spots in the country and prevented independent reporting, making it nearly impossible to verify accounts from either side as the conflict spirals out of control and turns increasingly violent.

Homs, which many refer to as "the capital of the Syrian revolution," has become a flashpoint of the nearly 11-month-old uprising against President Bashar al-Assad. Several neighbourhoods in the city, such as Baba Amr, are under the control of rebels.

On Saturday, Syrian forces killed up to 200 people in Homs — the highest death toll reported for a single day in the uprising — according to several rights groups. There was no way to independently confirm the toll.

'The regime is committing organized crimes.'—activist Majd Amer in Homs, Syria

While government forces have in the past used tanks and other weapons, the increased number of victims appear to have resulted from the indiscriminate use of artillery, according to the activists' reports.

"As of 6:30 this morning the shelling intensified with a rate of one shell every two minutes," Baba Amr activist Omar Sheker said during Monday's bombardment.

The uprising began with mostly peaceful protests against Assad, but government forces responded with a fierce crackdown. Now, army defectors and others are taking up arms to fight back, raising fears of civil war.

The threat of both sides turning to greater force increased Saturday when Russia and China vetoed a UN Security Council resolution aimed at ending the bloodshed. Now regime opponents fear that Assad will be emboldened by the feeling he is protected by his top ally Moscow and unleash even greater violence to crush protesters.

U.S. wrath

China said Monday it was forced to veto a United Nations vote on Syria because it was called before differences in the proposal were bridged, but denied playing spoiler and said it wants to see an end to violence there.

Also Monday, an explosion ripped through a gas pipeline in Homs, the state-run news agency, SANA, reported. SANA blamed terrorists. The regime says terrorists acting out a foreign conspiracy are behind the uprising, not protesters seeking change.

The Local Co-ordination Committees activist group said Monday's shelling in Homs hit a makeshift clinic in Baba Amr, causing casualties.

At least 17 people were killed across the city on Monday, according to the LCC and the Britain-based Syrian Observatory for Human Rights.

Activist Sheker said a paramedic was wounded in the shelling of the clinic and two people who were standing outside died instantly. He added that many volunteers at the hospital were wounded as well as people receiving treatment.

Syria's state-run TV denied government forces were besieging the area, saying activists in the city were setting tires on fire to make it appear as if there was a bombardment.

Syrian security forces are "chasing the terrorists and clashing with them," it said.

On Sunday, the commander of rebel soldiers said force was now the only way to oust Assad, while the regime vowed to press its military crackdown to bring back stability to the country.

"We did not sleep all night," Majd Amer, another activist in Homs, said by telephone. Explosions could be heard in the background. "The regime is committing organized crimes."

Amer said shelling of his neighbourhood of Khaldiyeh started at 3 a.m. local time, and most residents living on high floors either fled to shelters or to lower floors. He said electricity was also cut.

Demonstrations around the world

Protesters attacked seven Syrian embassies around the world after the UN Security Council failed to pass the resolution on Syria. About 40 men ransacked the Syrian embassy in the Australian capital of Canberra late Saturday. Protesters smashed windows and threw plants around the premises.

Earlier, demonstrators were able to break windows or force their way in at embassies in London, Athens, Berlin, Cairo and Kuwait.

A few dozen Canadians of Syrian descent protested in front of Syria's embassy in Ottawa, calling on the international community to intervene and end the violence in Syria.

Syria's largest opposition group, the Syrian National Council, said failure to pass the resolution has given the Syrian regime a "licence to kill" with impunity, leading to expectations the Assad government will now step up its efforts to stifle defiance wherever it finds it.

Canadian Foreign Affairs Minister John Baird said the failure by the council to effectively respond to the crisis in the Middle-Eastern country was "yet another free pass for the illegitimate Assad regime and those backing it."

"Those attempting to cling to power in Syria are morally bankrupt, and their disregard for human life is surpassed only by their cynicism over doing what is just and right," he said in a statement.

The Free Syrian Army, the armed rebels who have been increasingly active recently, has also vowed to step up its resistance against the Assad regime, which denied reports of the weekend assault in Homs, calling them part of a "hysterical campaign" of incitement by armed groups against the government.

The UN said in December that more than 5,400 people have been killed in Syria since March, but it has been unable to update its count for weeks due to recent chaos in the country.