Syrian forces shelled the town of Douma near the capital Damascus from dawn on Wednesday, while raking a village in the country's northwest with machinegun fire and killing a civilian, monitors said.
The Syrian Observatory for Human Rights said that residents of Douma, 13 kilometres (eight miles) northeast of Damascus, also reported bursts of gunfire in the town, where rebels have apparently sought refuge.
In Damascus itself, clashes erupted briefly between a security patrol and rebels in the Maisat district, but there were no casualties, the Britain-based watchdog said.
It added that one civilian was killed and three wounded in heavy machinegun fire by regime forces in Tell Ain al-Hamra, near the town of Jisr ash-Shugur in the northwest province of Idlib, on Wednesday.
"Jisr ash-Shugur is near the Turkish border and has had a significant presence of rebels since the beginning of the revolt," Rami Abdel Rahman, head of the Observatory, told AFP.
Rural villages east of Jisr ash-Shugur and the town itself came under mortar attack by security forces, he added.
In the same province, in the village of Ahsem, an army checkpoint was targeted by an explosion followed by gunfire, the Observatory reported.
Shots were also heard early Wednesday in several districts of the central Syrian city of Homs, a stronghold of opposition groups.
UN envoy Kofi Annan on Tuesday said his peace plan that went into effect April 12 but has since been regularly violated by both sides to the conflict was "probably the last chance to avoid civil war" in Syria.
Nearly 12,000 people, mostly civilians, have been killed in Syria since an anti-regime uprising erupted in mid-March last year, according to the Observatory.