Syria's prime minister has fled the country with his family after only two months in the post.
The departure of Riad Hijab for neighbouring Jordan is evidence that the widening cracks in president Bashar's Assad's regime have reached the highest echelons of government.
Mr Hijab - who planned the break for weeks, 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 US official said the defection was 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 of state security headquarters in the capital Damascus on July 18, including the defence minister and Assad's brother in law. There has been a steady 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, Mr Hijab's departure will not immediately undercut the regime's ability to fight rebels in places such as Aleppo. But such defections carry symbolic importance and highlight that dissent reaches into the upper levels of government.
Mr Hijab is part of Syria's Sunni majority, which forms the bedrock of the opposition in the 17-month-uprising that has claimed at least 19,000 lives.
Just hours before word of the defection got out, Assad suffered another blow in his attempt to portray he is in control: A bomb ripped through the third floor of the state TV building in Damascus, wounding at least three employees and displaying the ability of rebels to strike in the heart of the capital.
Mohammad Otari, Mr Hijab's spokesman, said the minister was appointed about two months ago and started planning his defection at that time. He said Mr Hijab asked rebels from the Free Syrian Army to help him escape, which they did. Mr Hijab left with his family and seven brothers, including two who held top government posts at the ministries of oil and environment