BEIRUT - In one of their deadliest attacks, Syrian army defectors killed at least 27 government soldiers and police Thursday in clashes in the south, activists said. The escalating political unrest prompted Canada to become the latest country to urge its citizens to leave Syria.

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

Attacks by army defectors have been escalating in recent weeks, raising concerns the country is headed toward civil war. Sanctions by Western powers and the Arab League have added to the growing pressure on Assad from within Syria.

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

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, because of fears of the government.

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

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

"The army was shelling the town with tanks shortly before the attack by insurgents occurred," Omar said.

Syrian troops are usually accompanied by policemen in buses whose job is to round up people after the army enters an area.

Citing witnesses on the ground, the British-based Syrian Observatory for Human Rights also reported clashes in Daraa province. Syria has sealed off the country to foreign journalists, making it impossible to confirm the accounts.

Amid the turmoil, Canadian Foreign Affairs Minister John Baird said Canadians should leave Syria as soon as possible while commercial flights are still available. Washington issued a similar warning for U.S. citizens in September.

The Obama administration is predicting Assad's downfall. State Department official Frederic Hof told Congress on Wednesday that Assad's repression may allow him to hang on to power, but only for a short time.

"Our view is that this regime is the equivalent of dead man walking," said Hof, the State Department's point man on Syria. He said Syria was turning into "Pyongyang in the Levant," a reference to the North Korean capital. He said it was difficult to say how much time Assad has left in power but added: "I do not see this regime surviving."

In an apparent bid to promote defections, Hof warned Syrian troops and Assad's top aides that their president may be setting them up for possible war crimes or criminal charges by claiming in an interview with ABC News last week that the army was not his to command.

"It's difficult to imagine a more craven disclaimer of responsibility," Hof told members of the House Foreign Affairs Committee. "Perhaps it is a rehearsal for the time when accountability will come."

The UN raised its death toll for the Syrian uprising substantially this week, saying more 5,000 people have been killed since the start. Assad's regime is growing more isolated with the mounting international sanctions to punish his regime for its bloody crackdown that has mostly targeted unarmed, peaceful protesters.

Turkey, once a close trade partner with Syria, has also imposed penalties, spoken out against Assad's crackdown and accepted Syrian refugees fleeing the bloodshed.

Turkey is also home to Syrian opposition figures and the commanders of the Free Syrian Army.

Syria's former ambassador to Sweden has also fled to Turkey to join the opposition, Syria's Foreign Ministry said Thursday. The ministry said in a statement he was fired in Stockholm in 2008 and accused him of profiteering, forgery, exploiting his post and fraud.

Also Thursday, Human Rights Watch issued a report alleging that dozens of Syrian military commanders and officials authorized or gave direct orders for widespread killings, torture and illegal arrests during the wave of anti-government protests.

The 88-page report by the New York-based group is based on more than 60 interviews with defectors from the Syrian military and intelligence agencies. It identifies 74 commanders and officials behind the alleged abuse.

"Defectors gave us names, ranks, and positions of those who gave the orders to shoot and kill, and each and every official named in this report, up to the very highest levels of the Syrian government, should answer for their crimes against the Syrian people," said Anna Neistat, associate director for emergencies at Human Rights Watch.

All of the defectors interviewed said their commanders gave standing orders to stop the overwhelmingly peaceful protests throughout the country "by all means necessary." They understood the phrase as an authorization to use lethal force, especially because they had been given live ammunition instead of other means of crowd control.

About half the defectors interviewed by HRW said the commanders of their units or other officers also gave them direct orders to fire at protesters or bystanders and reassured them that they would not be held accountable.

The report quotes defectors as saying that in some cases, officers themselves participated in killings. It said the abuses constitute crimes against humanity and that the UN Security Council should refer Syria to the International Criminal Court.