BEIRUT â€"  Activists said Syrian troops on Wednesday fired mortar shells and machine-guns at a Damascus suburb that was visited repeatedly by U.N. observers, prompting calls for the international mission to stay on in flashpoints to prevent regime attacks.

Activists say government troops have targeted opposition strongholds, such as the suburb of Douma, after monitors leave. They say the regime has halted shelling attacks in other areas where observers are deployed for longer periods.

Currently, 13 monitors are in Syria, but the team is to grow to 100 in a month and eventually to up to 300.

On Wednesday, regime forces pounded Douma for a second day, despite a visit by observers, said local activist Mohammed Saeed. He said regime forces fired before, during and after the visit, their third to Douma in three days.

Persistent bloodshed has tarnished efforts by observers to salvage a Syria truce that started to unravel almost as soon as it began on April 12. Despite the violence, the international community still sees the peace plan as the last chance to prevent the country from falling into civil war -- in part because there are no real alternatives.

Violence in Syria erupted more than 13 months ago, triggered by a brutal regime crackdown on what began as peaceful protests but gradually turned into an armed uprising.

The truce is intended to pave the way for talks between President Bashar Assad and those trying to bring him down on Syria's political future, but so far it appears to have only a limited impact on the violence.

Rebels have also kept up attacks. The state-run Syrian news agency SANA said a suicide bomber detonated a car packed with explosives near an army checkpoint along a highway in northwestern Syria on Wednesday, killing one member of the security forces and wounding two. Elsewhere, a roadside bomb went off in the northern Aleppo province, critically wounding three people, the agency said.

Syria restricts access to foreign journalists, and reports by activists and state media cannot be confirmed independently.

International envoy Kofi Annan told the U.N. Security Council on Tuesday that the situation in Syria is "bleak" and expressed alarm at continued regime attacks on towns where U.N. observers are not present. But he also expressed hoped that a speedy deployment of a 300-strong observer force authorized by the council could potentially "change the political dynamics" in Syria.

Opposition activists have said observers appear to make difference in areas where they stay for longer periods, such as the central city of Homs, where a pair of monitors has been deployed since the weekend. Homs had been hammered by regime artillery for weeks, but shelling stopped after the monitors arrived. Gunfights are still reported in some neighborhoods.

The city of Hama was also quiet Wednesday, a day after the U.N. said two observers were deployed there.

On Monday, after a brief observer visit to the city, more than 30 people were reported killed by regime forces in what activists portrayed as retaliation against those who dared to protest.

Local activist Mousab Alhamadee said a continued observer presence in Hama would embolden more people to demonstrate against the Assad regime.

"We usually go to demonstrations under shelling and fire," said the 30-year-old English teacher and translator. "Now, if we have the U.N. monitors with us, this will encourage us more, and we will go to our demonstrations in bigger numbers and hope the killing will stop."

Saeed, the Douma resident, said U.N. observers should also stay on in his neighborhood. So far, U.N. monitors have visited the suburb three times in three days. The Syrian news agency also reported that the observers were in Douma on Wednesday.

The suburb had come under heavy government attack on Tuesday, with at least eight people reported killed by shelling and heavy machine-gun fire. On Wednesday, government troops resumed their assault in the morning, firing tank cannon and heavy machine-guns, said Saeed. The Local Coordination Committees, a grassroots group of activists, reported bombardment with mortars and machine-guns.

Saeed said much of Douma has been without electricity for a day because of heavy damage caused by Tuesday's attacks. Residents are seeking cover from shelling on the lower floors of apartment buildings, he said. The streets are largely empty for fear of snipers on rooftops, he said. Among those killed Tuesday was a medical doctor killed by sniper fire while riding in an ambulance, he said.

An amateur video posted online from Douma Wednesday showed about 10 soldiers carring assault rifles and other weapons climbing out of the back of an armored vehicle while gunfire and explosions are heard in the background.

On Tuesday evening, rebel fighters firing rocket-propelled grenades in Douma killed a Syrian army officer and wounded three other troops, the Syrian news agency reported.