Human-rights activists and residents of Homs, Syria's third largest city, have reported intense gunfire as security forces conduct raids and arrests.

One resident told the Reuters news agency by phone that the army fired at worshippers in eastern Khalidiya district as they left the Khaled Ibn al-Walid mosque in the early hours of Thursday morning.

"There is heavy military deployment in Homs; military checkpoints are everywhere in the city. There is heavy shooting in Bab al-Sebaa, one house was burned and the humanitarian conditions are dire," the Syrian Observatory for Human Rights, a London-based independent rights monitor, said in a statement.

Activists say more than 50 people have been killed in the city since the weekend.