As Syrian rebels recaptured a string of Aleppo neighbourhoods, America's top military official said the end is nigh for President Bashar al-Assad's beleaguered regime.

The armed rebel groups, led by the Free Syrian Army (FSA), have gained control over swathes of districts in Aleppo, Syria's commercial capital, following bitter battles with pro-Assad forces.

"If they [government troops] continue this kind of tragic attack on their own people in Aleppo, I think it ultimately will be a nail in Assad's own coffin," US Defence Secretary Leon Panetta told reporters before embarking on a week-long trip to the Middle East and Africa.

"What Assad has been doing to his own people, and what he continues to do to his own people, makes clear that his regime is coming to an end. It's lost all legitimacy. It's no longer a question of whether he's coming to an end, it's when," Panetta added.

During his visit, Panetta will hold talks with officials from Jordan, Tunisia, Egypt and Israel. He says he will use the talks to insist that Assad steps down as president, and his departure precipitates a peaceful transition to democracy in Syria.

Panetta will also emphasise that Syria's stockpile of chemical weapons must not end up in the wrong hands.

There is still no let up in clashes in Aleppo, with Syrian forces continuing to pound rebel positions using helicopter gunships and artillery. The Syrian regime has pledged on state television that the rebels will be driven away and Aleppo will return to safety.

"Life in Aleppo has become unbearable. I'm in my car and I'm leaving right now. There's shelling night and day, every day. Bread, gasoline and gas are being sold on the black market at very high prices. Many things are in shortage," a resident in Aleppo told the Associated Press on condition of anonymity.

The UN Undersecretary-General for Humanitarian Affairs, Valerie Amos, said more than 200,000 people have already fled Aleppo, Syria's most populous city, to save their lives.

"Many people have sought temporary shelter in schools and other public buildings in safer areas. They urgently need food, mattresses and blankets, hygiene supplies and drinking water," Amos said.

Amos urged both the government and rebel forces to refrain from targeting civilians, and allow humanitarian organisations safe access.