The governor of Homs has said a deal has been reached with the United Nations to evacuate civilians from the besieged old city in Syria.

Forces loyal to Syrian President Bashar al-Assad are surrounding rebel-held areas.

Earlier, Russia's Foreign Ministry said that an agreement to allow in humanitarian aid was also under discussion.

Meanwhile, the Syrian Observatory for Human Rights said rebels have seized control of most of Aleppo's central prison, freeing hundreds of detainees.

Observatory director Rami Abdel Rahman said the rebel groups had taken control of 80% of the prison.

However, state media denied the report, saying soldiers and security forces had "thwarted an attack against the prison by terrorist groups".

A coalition of Syrian rebels earlier announced a new military operation in Aleppo province, where more than 250 people have been killed in alleged regime barrel bomb attacks since Saturday.

The Islamic Front - a huge alliance grouping tens of thousands of rebels - and the jihadist Al-Nusra Front announced an operation dubbed "Truthful Promise Approaches".

The rebel group asked residents of "occupied areas" to stay away from government checkpoints and bases, saying they would be targeted.

The announcement comes as the Syrian army seeks to take territory in the rebel-held eastern part of Aleppo city.

Troops are moving from areas around Aleppo international airport after recapturing territory nearby and reopening it to air traffic.

The ground campaign has been accompanied by six consecutive days of aerial attacks involving explosive-packed barrel bombs dropped from army helicopters.

The Syrian Observatory for Human Rights said  that at least 257 people have been killed in barrel bomb attacks on eastern Aleppo since Saturday, including 11 on Thursday.

The dead include at least 76 children, according to the British-based group.

Hundreds more have been wounded in the raids using the controversial unguided munitions, which have been condemned by rights groups as indiscriminate.

The heavy casualty toll has sparked a mass exodus from the worst-hit neighbourhoods in the east of the city.

Once the country's economic hub, Aleppo has been divided between government control in the west and rebel control in the east since a rebel offensive in mid-2012.

The bombing in the east has prompted thousands of civilians to flee across the nearby border to Turkey, or seek refuge in the government-held part of Aleppo.