Syrian forces renewed their bombing of parts of the shattered city of Homs and for a second day blocked Red Cross aid meant for civilians stranded without food and fuel in the former rebel stronghold, activists and aid workers said.

The government assault came a day after United Nations Secretary-General Ban Ki-moon said he had received "grisly reports" that President Bashar al-Assad's troops were executing and torturing people in the city after rebels abandoned their positions there.

Syria's government says it is fighting foreign-backed "terrorists" whom it blames for killing hundreds of soldiers and police across the country.

The UN says Syrian security forces have killed more than 7,500 civilians since a revolt against Assad's rule began in March last year.

Concern was mounting for civilians in freezing conditions in Baba Amr, where International Committee of the Red Cross (ICRC) trucks were being held up by Assad's forces.

Anti-government activists said they feared troops wanted to prevent the ICRC witnessing a reported massacre of rebels in Baba Amr, which had become a symbol of the year-long uprising.

A Damascus-based ICRC spokesman said Syrian authorities had given the convoy permission to enter but government forces on the ground had stopped the trucks because of what they said were unsafe conditions, including "mines and booby traps".

The outside world has proved powerless to halt the killing in Syria, where repression of initially peaceful protests against Mr Assad's rule has spawned an insurrection by army deserters and others.

Russia and China have twice vetoed council resolutions that would have condemned Damascus, accusing Western and Arab nations of pushing for Libya-style "regime change" in Syria.

China urged both Damascus and the rebels to end the violence immediately and start talks, but again said it opposed any foreign military intervention in Syria.