BEIRUT — Syria’s prime minister defected and fled to neighboring Jordan, a Jordanian official and a rebel spokesman said today, evidence that the widening cracks in President Bashar’s Assad’s regime have reached the highest echelons of government.

Riad Hijab, who planned the break for months, according to an aide, is the highest-level political figure to switch sides and is certain to encourage rebels after a string of military and diplomatic figures abandoned the regime.

A senior U.S. official said the defection is more evidence that the Assad regime “is crumbling.”

The Syrian regime has suffered a series of significant setbacks over the past month that point to a loosening of its grip on the country.

Four of the president’s top security aides were killed in a rebel bombing July 18, including the defense minister and Assad’s brother in law. There has been a stream of high-level defections from diplomats to generals.

And the regime has been unable to fully subdue rebel challenges in the two major cities, Damascus and Aleppo.

But power remains closely held within Assad’s inner circle and even posts such as the prime minister have limited clout.

Because he is not part of that elite, Hijab’s departure will not immediately undercut the regime’s ability to fight rebels in places such as Aleppo, Syria’s largest city, which has been pounded by gunners and warplanes.