BEIRUT, Lebanon (AP) — United Nations monitors toured a rebel-held town in central Syria on Sunday with army defectors, while government troops pounded a Damascus suburb with artillery and heavy machine guns, opposition activists said.

The shelling in the suburb, Douma, highlighted the need for more observers a day after the Security Council voted to expand their number to 300 from 30 in hopes of salvaging an ostensible truce marred by continued fighting between the military and the rebels.

An eight-member team is already on the ground in Syria, and since Thursday it has visited flashpoints of the 13-month conflict. Fighting generally halts temporarily when the observers are present, but there has been a steady stream of reports of violence from towns and regions where they have not yet gone.

“This U.N. observers thing is a big joke,” said one Douma-based activist, Mohammed Saeed. “Shelling stops and tanks are hidden when they visit somewhere, and when they leave, shelling resumes.”