The head of the UN observers in Syria has said a spike in violence is derailing the mission, which is the only functioning part of an international peace plan to calm the country's spiralling crisis.
Major General Robert Mood on Friday blamed both sides of the conflict for the escalating bloodshed.
"Violence over the past 10 days has been intensifying willingly by both parties, with losses on both sides and significant risks for observers," Mood told reporters in Damascus.
He said the escalating violence is now 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 yet that a peace plan brokered by international envoy Kofi Annan two months ago is disintegrating.
The regime and the opposition have both ignored a truce that was supposed to go into effect on April 12.
The government kept up a ferocious offensive on rebel areas around the country on Friday 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 al-Hiffa 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 Hiffa, a Sunni-populated village, had become a focus of international concern because of fears the uprising against President Bashar al-Assad is 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.
UN observers have reported a steep rise in violence and a dangerous shift in tactics by both sides in Syria in recent weeks.
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.
Also on Friday, Human Rights Watch (HRW) reported that Syrian government forces have used sexual violence to torture men, women, girls and boys detained during the current conflict,
The New York-based group said it had interviewed 10 former detainees, including two women, who described being sexually abused or witnessing such abuse in detention.
In-depth coverage of escalating violence across Syria
This included "rape, penetration with objects, sexual groping, prolonged forced nudity and electroshock and beatings to genitalia", a statement said.
Many witnesses said they were imprisoned because of their political activism, while others said the reasons for detention were unclear but that detainees suffered the same abusive tactics.
"Sexual violence in detention is one of many horrific weapons in the Syrian government's torture arsenal and Syrian security forces regularly use it to humiliate and degrade detainees with complete impunity," said Sarah Leah Whitson, HRW's Middle East director.
Activists say about 14,000 people have been killed since the uprising against Assad began in March 2011.
State-run news agency SANA reported that authorities had arrested an al-Qaeda terrorist who planned to blow himself up in a Damascus mosque during Friday prayers. It identified the man as Mohammad Hussam al-Sudaqi.