Syria's main opposition group called on Thursday for an emergency UN meeting after the reported killing of more than 100 people in the city of Hama, as Russia blamed rebels for stoking the unrest.
The Syrian National Council appeal cast further doubt on a putative ceasefire that technically went into effect on April 12 but has failed to take hold amid persistent deadly violence.
"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 SNC said.
"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," a statement added.
The Security Council must be ready to order sanctions if Syria flouts commitments to halt violence, US ambassador to the UN Susan Rice said on Thursday.
"We condemn what remains the government's refusal to abide by its commitments, its continued intense use of heavy weaponry in Hama and elsewhere, which continues to result in large numbers of civilian deaths every day," she told reporters.
Arab League Secretary General Nabil al-Arabi called for observers to be deployed rapidly.
"The entire world is waiting for a truce and the observers to be deployed, but unfortunately the fighting has not stopped and every day new victims die," he told a league ministerial meeting in Cairo.
"The important thing now is the ceasefire, and this will only happen if a sufficient number of observers is deployed."
The truce brokered by UN-Arab League envoy Kofi Annan is to be monitored by 300 UN observers due to arrive in Syria in the coming weeks.
A small advance team is already on the ground, and some members have set up base in Hamas as well as the nearby city of Homs.
Monitors said regime shelling of a working-class district of Hama killed at least 12 people on Wednesday and flattened a block of houses, but activists put the death toll as high as 68, including 16 children.
State news agency SANA said 16 people were killed when a bomb "terrorists" were preparing exploded prematurely inside a house in the central city.
At least 40 people were also reportedly killed in Hama on Monday, including nine activists "summarily executed" after meeting UN monitors, a rights group said.
The reports cannot be verified because of restrictions on foreign media.
Moscow, a long-time Damascus ally, blamed the violence on rebel forces and hinted at Al-Qaeda involvement in the revolt against President Bashar al-Assad's regime.
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"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.
On Wednesday, France raised the prospect of military intervention if Annan's peace plan fails.
Without quick progress, Foreign Minister Alain 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.
However, Juppe added 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.
Moscow on Thursday dismissed the French calls for force.
"It's a counterproductive approach," Russian news agencies cited deputy foreign minister Mikhail Bogdanov as saying.
"It seems to us the only way to avoid civil war, the most promising method, is a national dialogue."
At least seven people were killed nationwide on Thursday, five of them civilians, according to the Syrian Observatory for Human Rights.
Damascus blamed the continued unrest on "armed terrorist groups."
"Armed terrorist groups have intensified (the number of) massacres, explosions and acts of aggression, committing more than 1,300 violations since the ceasefire came into force on April 12," Information Minister Adnan Mahmoud told AFP.
In a sign of growing frustration with Assad's regime, Turkey said it was considering all possibilities if the unrest continues and the Council of Europe urged the Security Council to impose an arms embargo on Syria.
"In the face of developments in Syria, we are taking into consideration any kind of possibility in line with our national security and interests," Foreign Minister Ahmet Davutoglu told parliament in Ankara.
"This is not an intervention or warmongering as some claim."
Meanwhile, top rebel military leader General Mustafa al-Sheikh said the UN monitoring mission is doomed to fail because Assad will never comply with a ceasefire, pan-Arab daily Asharq al-Awsat quoted him as saying.
More than 9,000 people have died since the revolt erupted in March 2011, the United Nations says, while NGOs put the figure at more than 11,100.