Syrian security forces are carrying out deadly attacks on opposition-held areas in the north and south and continuing to bombard Homs, activists say.
Some opposition groups report that more than 50 people have been killed over the course of the day.
The violence came as envoy Kofi Annan travelled to Qatar to brief the Arab League on the peace plan he negotiated.
A handful of United Nations observers have made their first field trip to the south of the country to visit Deraa.
Earlier, the head of the advance team of UN observers admitted its ceasefire monitoring mission would be difficult.