The main Syrian opposition group says Homs is a disaster area and is urging international protection for civilians.
The Syrian National Council said government forces had been shelling residential areas for days.
It says civilians are very short of food and medical supplies, and electricity and water have been cut.
Last week the Syrian government agreed to an Arab League deal to pull troops out of residential areas.
However, the BBC’s Jonathan Head, in neighbouring Turkey, says this has not happened.