BEIRUT, Lebanon - Army defectors killed 27 government forces Thursday in apparently coordinated attacks that were among the deadliest by rebel troops since the uprising began nine months ago. The escalating unrest prompted Canada to advise thousands of its citizens in Syria to leave.

The fighting began around daybreak in the southern province of Daraa, where the uprising against President Bashar al-Assad's authoritarian regime began in March.

Syria has seen a sharp escalation in armed clashes recently, raising concerns that the country of 22 million is headed toward civil war. The United Nations raised its death toll this week, to more than 5,000 since the revolt began.

"The attacks by army defectors are becoming more coordinated and more deadly," said Mohamad Bazzi, a Syria expert at the Council on Foreign Relations. "Unfortunately, this will likely lead to a new cycle of escalation by the regime."

Hanging on

Sanctions by Western powers, Turkey, and the Arab League have added to growing pressure on Assad from within Syria. U.S. State Department official Frederic Hof said Wednesday that Assad's repression may let him hang on to power, but only for a short time.

Still, the regime could exploit the escalation of armed attacks by military defectors to escalate the crackdown with full force on pockets of defectors concentrated in Daraa and the northwestern province of Idlib, which borders Turkey.

Defectors from the Free Syrian Army, whose leaders are in exile in Turkey, fired a rocket-propelled grenade Thursday at a bus carrying police into the town of Busra al-Harir, killing 12 officers, an activist said.

That set off clashes with an accompanying force of soldiers. The defectors killed 13 of them, said the activist, who would only agree to be identified by his first name, Omar, for fear of retribution.

The fighters then killed two more soldiers in an attack on a checkpoint, he said.

Russian measure

Busra al-Harir is home to about 300 army defectors who have been clashing with regime forces daily for nearly a week, he said.

Syrian troops are usually accompanied by policemen in buses, who round up people after the army enters an area.

At the United Nations, Russia began circulating a draft resolution to resolve the conflict. It calls for an end to all violence. Russia has criticized opponents of Assad's rule for employing violent tactics. Western nations said the Russian draft did not go far enough.

Assad has denied issuing orders to kill protesters. But New York-based Human Rights Watch said in a report that dozens of military commanders and officials authorized or gave direct orders for widespread killings and torture.