The defection Monday of Syrian Prime Minister Riad Hijab and his announcement that he was joining the rebel forces was a major coup for the anti-government movement fighting to oust President Bashar al-Assad, U.S. officials and analysts said.

Hijab, who fled to Jordan with his family and several other Syrian officials, said that he was leaving Assad's "killing and terrorist regime" for "the ranks of the freedom and dignity revolution." He planned his defection for more than two months with the help of the Free Syrian Army, an opposition force made up of Syrian military defectors and civilian rebels.

James Carafano, a senior defense analyst with the Heritage Foundation, said the highest-ranking defection from Assad's regime signals to the world that even those closest to Assad have doubts about whether he'll remain in power.

"It does seem the [Free Syrian Army and opposition forces] have reached a point they can conduct sustained campaigns," Carafano said. "So win, lose or draw in individual battles, they have demonstrated they can sustain a civil war. This reality is likely reflected in the recent spate of high-level defections from the government."

A U.S. official with direct knowledge of the ongoing battle in Syria said the mounting defections from Assad's regime could take a psychological toll on the government.

"The impact of defections on regime stability will depend in part on the role these people play in the opposition," the official said. "And with the heat on the regime rising, Assad may be starting to wonder about the loyalty of some in his inner circle."

More than 19,000 people have been killed in Syria since the anti-government uprising began in March 2011, and the intense fighting in Aleppo, the nation's largest city, is only adding to the death toll.

"Assad cannot fight us forever," said a Syrian activist who spoke to The Washington Examiner via Skype. The activist requested anonymity, fearing for his personal safety. "The people will continue to fight for freedom. Assad will fall."

The anti-government rebels are gaining in strength and "even though the rebels aren't a unified fighting force, there's every reason to think they have the wherewithal to sustain a conflict with the regime for some time yet," the U.S. official said.

Assad, who has been in hiding for several weeks, is finding it more difficult to sustain his power despite his military's repeated attacks on the rebel forces. Monday's defections raised further questions about the strength of Assad's position.

"We view the defection of the head of Assad's government as a strong signal that his support is rapidly unraveling," said Abdullah Ibraheem of the Syrian Expatriates Organization, "including the ones assumed to be the loyalists."

Sara A. Carter is The Washington Examiner's national security correspondent. She can be reached at