The head of the UN observers in Syria says a spike in violence is derailing the monitoring mission, which is the only functioning part of an international peace.
Major General Robert Mood blamed both sides for the escalating bloodshed.
"Violence over the past 10 days has been intensifying ... with losses on both sides and significant risks for observers," Mood told reporters in Damascus on Friday.
He said the escalating violence was limiting the mission's ability "to observe, verify, report as well as assist in local dialogue and stability projects".
Mood's comments were the clearest sign that a peace plan brokered by international envoy Kofi Annan two months ago was disintegrating.
On Friday, the Syrian regime kept up a ferocious offensive on rebel areas around the country in one of the most serious escalations in violence since Annan brokered the truce.
An activist in the northern city of Aleppo said troops backed by helicopters and tanks were engaged in "raging battles" in the rebel-held town of Anadan and several other locations in the province.
Syrian troops have been sweeping through villages and towns in Syria's northern, central, southern and seaside provinces this week.
The military on Wednesday overran the town of Haffa in the coastal Latakia province, pushing out hundreds of rebels after intense battles that lasted eight days. UN observers entered the nearly deserted town on Thursday and found smoldering buildings, looted shops, smashed cars and a strong stench of death, according to UN spokeswoman Sausan Ghosheh.
The siege of Haffa, a Sunni-populated village, had become a focus of international concern because of fears the uprising against President Bashar Assad was evolving into a sectarian civil war pitting his minority Alawite sect against the majority Sunnis and other groups.
Recent mass killings in other Sunni-populated areas have fed those concerns.
In other parts of Syria, more than 40 civilians and opposition fighters were killed on Thursday, according to activists, alongside more than a half-dozen Syrian forces.
The fighting included clashes in the town of Hamuriya, near Damascus, that killed at least nine men who were allegedly butchered with knives. A video circulated by activists showed a pile of lifeless men, including one who was clearly slashed through the neck.
UN observers have reported a steep rise in violence and a dangerous shift in tactics by both sides. Car bombings and suicide bombings have become increasingly common as the 15-month uprising against Assad becomes militarised.
Most have targeted security buildings and police buses, symbols of Assad's regime.
Activists say some 14,000 people have been killed since the uprising against Assad began in March 2011.
State-run news agency SANA reported Friday that authorities had arrested an al-Qaeda militant who planned to blow himself up in a Damascus mosque during Friday prayers. It identified the man as Mohammad Hussam al-Sudaqi.
Russia has denied a statement by US State Department spokeswoman Victoria Nuland that Moscow and Washington were discussing a post-Assad transition strategy.
"It's not true that we are discussing Syria's fate after Bashar Assad," Lavrov said.
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