Syrian army defectors killed at least 27 government forces in clashes in the southern province of Daraa on Thursday, activists said. It was one of the deadliest spates of attacks by rebel troops since the uprising against President Bashar Assad's authoritarian regime began nine months ago.

Citing witnesses on the ground, the Britain-based Syrian Observatory for Human Rights said three separate clashes erupted at dawn in Daraa, where the uprising 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.

There was no immediate claim of responsibility for Thursday's attacks. But the Free Syrian Army, a Turkish-based defector group, has in the past claimed similar attacks.

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 U.N. 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.

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 U.N. Security Council should refer Syria to the International Criminal Court.

Assad's regime claims armed gangs and terrorists are behind the uprising, not protesters seeking more freedoms and reform in one of the most totalitarian regimes in the Middle East.

In the ABC interview with Barbara Walters, Assad claimed he never ordered the brutal suppression of the uprising, even though he commands the armed forces.

The government has sealed off the country to most outsiders while clinging to the allegation that the uprising is the work of foreign extremists, not true reform-seekers aiming to open the authoritarian political system.

The United Nations and others observers dismiss that notion entirely, blaming the regime for widespread killings, rape and torture. Witnesses and activists inside Syria routinely describe brutal repression, with government forces firing on unarmed protesters and conducting house-to-house raids in which families are dragged from their homes in the night.

"Try as he may to distance himself from responsibility for his government's relentless brutality, President Assad's claim that he did not actually order the crackdown does not absolve him of criminal responsibility," said Neistat of Human Rights Watch. "As the commander in chief of the armed forces, he must have known about the abuses _ if not from his subordinates, then from U.N. reports and the reports Human Rights Watch sent him."