Syria's main opposition group on Thursday called for an emergency UN Security Council meeting as it accused regime forces of having killed more than 100 people in the central city of Hama.
The appeal came as Russia accused rebels of terror attack and France raised the prospect of military action to halt violence in Syria, where President Bashar al-Assad's forces have waged a bloody year-long crackdown on dissent.
"We are calling for an emergency meeting of the UN Security Council so that it can issue a resolution to protect civilians in Syria," the Syrian National Council said in a statement.
"Hama in recent days, and following a visit by UN observers, witnessed a series of crimes... that left more than 100 people dead and hundreds wounded because of heavy shelling.
The Syrian Observatory for Human Rights monitoring group said regime shelling of a working-class district of Hama killed at least 12 people on Wednesday, but activists on the ground put the death toll as high as 68, including 16 children.
State news agency SANA said at least 16 people were killed, including women and children, when a bomb that "terrorists" were setting up went off prematurely inside a house in the city.
At least 31 people were reportedly killed during shelling of a different neighbourhood on Monday.
Also, according to the Syrian League for Human Rights, regime forces "summarily executed" nine activists on Monday, a day after they had met UN observers overseeing a fragile ceasefire that went into effect 12 April.
In what it called a sign of Al-Qaeda involvement in the revolt, Russia on Thursday accused Syrian rebels of waging a terror campaign designed to kill as many civilians.
"Opposition groups have essentially reverted to waging wide-scale terror in the region," said Russian foreign ministry spokesman Alexander Lukashevich.
Attacks aimed at "killing as many peaceful civilians as possible and destroying civilian infrastructure remind one of what is happening in Iraq, Jordan and other places where Al-Qaeda and its groups operate," he said.
The truce brokered by UN-Arab League envoy Kofi Annan has failed to take hold with unrest and killings reported on a daily basis in various parts of the country.
At least two people were killed on Thursday, monitors said, as clashes were reported in several provinces.
Loud blasts and heavy gunfire could be heard in Harasta, a suburb of Damascus, the Observatory said, adding regime troops carried out raids between Harasta and Barzeh, a district of the capital.
French Foreign Minister Alain Juppe on Wednesday said that given the persistent violence, the UN-backed peace plan was "seriously compromised" and held out the threat of seeking military action to end the regime's crackdown.
Annan had urged a rapid deployment of the full, 300-strong observer team agreed by the Security Council, and Juppe said they should be on the ground within a fortnight.
Without quick progress, Juppe said the international community would have "to move on to another step which we have already started raising with our partners, under Chapter Seven of the United Nations charter."
A Chapter Seven resolution authorises foreign powers to take measures including military options.
Juppe pointed out however that such a resolution, which was also mooted by US Secretary of State Hillary Clinton last week, was unlikely to pass, alluding to previous Security Council vetoes by Russia and China.
At the moment, there is an advance team of UN observers in the country set to number 30 by the end of the week.
Juppe said that May 5, when Annan is to present his next report on the peace process, would be "a moment of truth."
If the UN mission "is not working, we cannot continue to accept the defiance of the regime" and the international community will have "to move on to other things to stop the tragedy."
On Tuesday, former UN chief Annan told Security Council that he was "concerned" about violence surging after members of the advance team visit individual cities.
The unrest began as a popular revolt but has turned into an insurgency that many fear could lead to all-out civil war. More than 9,000 people have died since the revolt broke out in March 2011, according to the United Nations.