“It’s a beginning of an endgame sort of thing,” he added.

Mr. Hijab’s departure came less than a month after four members of Mr. Assad’s inner circle were killed in a bomb attack in Damascus that raised serious questions about the cohesiveness of the embattled government. On Monday, rebels struck again close to the leadership’s core, bombing the third floor of the government television and radio headquarters, which have been used to reassure the population that Mr. Assad remains in control.

No one died this time, but the explosion — shown on Syrian television, where officials insisted it was insignificant — again highlighted the rebels’ ability to breach government institutions.

Defections highlight another vulnerability: betrayal within the ranks of supposed loyalists. Over the past few months, there has been a steady flow of high- and midlevel figures announcing that they have turned on the regime. In recent days, in addition to Mr. Hijab, Syria’s most famous astronaut, an air force officer named Ahmed Faris, fled to Turkey, pledging his loyalty to the opposition.

In Washington, the White House spokesman, Jay Carney, said the defections were “a sign that Assad’s grip on power is loosening.”

“That the titular head of the Syrian government has rejected the ongoing slaughter being carried out at Assad’s direction only reinforces that the Assad regime is crumbling from within and that the Syrian people believe that Assad’s days are numbered,” he said.

Rebel leaders and defectors said that the process for leaving varied. In some cases, military officers have taken their allotted leave and have never returned to their units. Other defectors say they have falsified paperwork or used disguises to get through government checkpoints. In June, a Syrian Air Force pilot simply landed his fighter jet at an airport in Jordan.

Most of the defectors have been members of the Sunni majority, breaking away from a government dominated by Mr. Assad’s Alawite minority. Mr. Hijab, who has served in government for most of his life after receiving a Ph.D. in agriculture, is typical. The well-educated head of a Sunni family drawn into government by Mr. Assad’s father in an effort to add legitimacy to his government, he benefited from the government’s patronage before finally rejecting it.