Violence continued across the country, and activists said at least 300 people had been killed Friday in the capital city of Damascus, The Guardian of Britain reported.
The rebel Free Syrian Army said it had captured two border crossings between Syria and Turkey and one in Iraq, and refugees fled into neighboring countries.
Government forces were fighting to regain control of key suburbs of Damascus, while the army appeared to have taken control of the central district of Midan, where opposition sources said aircraft and helicopters had attacked targets on the ground.
The FSA said it had withdrawn from the district amid heavy bombing.
"Our heroic forces have completely cleansed the Midan area of the terrorist mercenaries," Syrian state TV said.
The Guardian reported the government appeared to be losing control of large parts of the country. President Bashar Assad's forces still held key cities by day, but seem to be becoming more vulnerable to guerrilla raids, the newspaper said.
Diplomats said Assad had phoned Maj. Gen. Robert Mood, the head of the U.N. monitoring mission, after Wednesday's bomb attack in Damascus killed four members of the Assad regime, and said Syria would implement U.N.-Arab League special envoy Kofi Annan's peace plan.
Neither Assad nor rebels have observed the cease-fire, the United Nations said.
Record numbers of Syrians are trying to escape the country, and 30,000 fled into Lebanon in a 48-hour span, the U.N. refugee agency said.
The New York Times reported video uploaded Friday appeared to show an attack on protesters in Damascus and fighting between rebels and government forces in Aleppo.
Clips showed protesters rallying outside police headquarters in Damascus, then fleeing gunfire.
Other clips showed bodies of victims said to have been killed in the attack on protesters in Damascus and one clip was said to show government forces and a tank on the streets of a neighborhood in Aleppo.
The United Nations estimates more than 10,000 people have been killed since the crisis in Syria began in March 2011. Opposition leaders have said the death toll is closer to 15,000.