Fighting within Damascus city is "the fiercest yet" residents have told Reuters news agency.
There have been many reports of loud explosions and persistent gunfire in south Damascus.
UN observers toured Tremseh for a second day as the Syrian government denied claims that its forces killed more than 100 people on Thursday.
An attack in Tremseh in Syria appeared to target the homes of army defectors and activists, a spokesman for the UN observers in Syria said.
The international community has come together in its condemnation of the Syrian regime after more than 200 civilians were killed in Hama.