BEIRUT — Syria’s prime minister began planning his break from the regime two months ago when Bashar Assad offered him the post and an ultimatum: Take the job or die.

The full scope of Riad Hijab’s carefully executed flight to the rebel side — described by an aide who escaped with him to Jordan — reverberated Monday through Syria’s leadership. Hijab became the highest-ranking government official to defect, emboldening the opposition and raising fresh questions about the regime’s ability to survive the civil war.

Although Assad has been hit by a string of embarrassing defections of military and political figures, they have yet to cause visible changes in the regime’s abilities on the battlefield. The loss of high-profile government officials, however, suggests fissures are reaching deeper into the ruling system and could force Assad to retreat further behind a cadre of loyalists as fighting flares on several fronts.

“Every defection is another door closed for Assad and another one open for the rebels,” said Mustafa Alani, an analyst at the Gulf Research Center based in Geneva. “It may not be the tipping point for the regime, but each breakaway is another crack.”

Hijab and an entourage of family members were expected to head next to the Gulf state of Qatar, a key backer of the Syrian rebels, in a further sign of the regional brinksmanship and gambits over Assad’s fate. Gulf states and Turkey have strongly backed the rebel forces while Assad has counted on support from a dwindling list of allies such as Iran and Russia.

Ahmad Kassim, a senior official with the rebel Free Syrian Army, initially said Hijab defected along with three other ministers, then later said two other ministers had left. However, Hijab’s spokesman, Mohammad Otari, said he fled the country alone.

Hijab’s defection is a humiliating blow for Assad after a string of generals and ambassadors has peeled away. Like nearly all prominent defectors so far, Hijab is a member of Syria’s majority Sunnis — the Muslim sect which forms the bedrock of the more than 17-month uprising.

His break suggests that elements of the Sunni elite — long a pillar of Assad’s rule — could be growing uneasy with the relentless bloodshed and the hardline policies of Assad’s minority Alawite community, which dominates the regime’s inner circle. The Alawite sect is an offshoot of Shiite Islam.

Secretary of State Hillary Rodham Clinton is expected to arrive in Turkey later this week for meetings on Syria.