The city of Raqqa, home to half a million people before the uprising against President Bashar Assad began two years ago, could be a test case for how rebels run areas they capture.

The rebel groups that led the battle for the city are comprised largely of strongly conservative Muslims, some of them extremists, and videos released over the weekend indicate some fighters have killed captured soldiers.

Recent government airstrikes, meanwhile, show the limits of rebel control. Even if they hold the ground, they can do little about the government's air force, which often bombards areas recently captured by the rebels. The regime also regularly accuses the rebels of attacking civilians.

Also yesterday, some of the fiercest fighting in a year was reported in Baba Amr, the neighbourhood in the central city of Homs that had stood for rebel defiance but also for the government's ability to strike back.

The Syrian military besieged Baba Amr for a month last year, killing hundreds of people, and eventually retook the area.

Yesterday rebels and regime troops clashed in Baba Amr, accompanied by army shelling and airstrikes, said the Britain-based Syrian Observatory for Human Rights.