Syrian rebel fighters in Homs have launched an attack on government forces in a bid to reclaim the city, which they say has been under siege for 69 days.

In a video uploaded to a social media website, rebels were seen under sustained attack while they fired rocket-propelled grenades at what they said were the positions of government forces.

"August 13 2012, in the name of God, today God willing will be the beginning of the lifting of the siege on the city of Homs," said a man who stood alongside the rebels, talking into the camera.

He added: "Homs has been under siege for 69 days and this is the start of the real battle."

The rebels' assault on Homs intensified after the focus in the protracted Syrian conflict shifted to Aleppo where rebels fighting to oust President Bashar al-Assad have fought lengthy street battles.

The rebels, who seized parts of the city three weeks ago, have been fighting to hold their ground against Asaad troops who are backed by warplanes, helicopter gunships, tanks and artillery.