Russia and Turkey meanwhile dismissed growing calls led by US President Barack Obama for Assad to quit, offering the embattled Syrian leader rare support despite a damning UN report on his "apparent shoot to kill" policy.
But the European Union was preparing sanctions against Syria's key oil sector, a European diplomatic source told AFP.
On the political front, a group of "revolutionary blocs" formed a coalition vowing to bring down the regime and paid tribute to more than 2,000 civilians killed in crackdown on protesters since mid-March.
Activists said that that 19 protesters were killed and many others wounded in demonstrations after Friday prayers.
The Syrian Observatory for Human Rights said 11 people, including an 11-year-old and a 72-year-old, were killed in the southern province of Daraa, epicentre of the anti-regime protests that erupted March 15.
Three others were killed in the central city of Homs and one in the Harasta suburb of Damascus.
The Observatory said security forces opened fire on protesters, also wounding 16 people, in the Ghabagheb, Inkhil, Al-Herak and Nawa in Daraa, but the official SANA news agency blamed the shooting on "armed men."
The agency said a policeman and a civilian were killed in Ghabagheb and six security forces wounded.
Tens of thousands of people flooded streets in major Syrian towns on Friday as they emerged from the weekly Muslim prayers, with the largest anti-regime demonstration reported in Homs.
