Shelling by Syrian forces in the central province of Hama has killed 34 people according to the British-based group the Syrian Observatory for Human Rights, BBC News and Reuters reported Sunday.
Hama has been a focal point of Syria's 14-month uprising against President Bashar al-Assad.
In a separate incident, there was an explosion near a convoy carrying the head of the UN mission in Syria.There are no reports of casualties in that case and it is not clear whether it was a bomb or a rocket-propelled grenade.
A gunbattle has erupted in Beirut as fears mount the conflict in neighbouring Syria is bleeding across the border.
Six people were wounded in the clashes that erupted late Sunday and continued early Monday, Lebanon's National News Agency says. The fighting came hours after Lebanese soldiers fatally shot an anti-Syrian cleric and his bodyguard when they failed to stop at a checkpoint. The Lebanese army said it deeply regretted the incident and that a committee will investigate.
An Associated Press reporter at the scene says the clashes involve pro- and anti-Syrian groups.
The explosive device was detonated about 150 metres from visiting UN peacekeeping chief Herve Ladsous and Maj. Gen. Robert Mood, the chief of UN observers in Syria. Journalists accompanying the team also were nearby. The explosion blew off the front of a parked vehicle.
A UN observer team with more than 250 members now on the ground has failed to quell the bloodshed in Syria, although it says it has had a "calming effect" in certain areas. On several occasions, the team has come close to being caught in an attack, although there is no conclusive proof that it has been targeted.
Earlier this month, a bomb targeting an army truck exploded seconds after a convoy carrying Mood went past in the country's south. Last week, a roadside bomb damaged the mission's cars in a northern town just minutes after witnesses said regime forces gunned down mourners at a funeral procession nearby.
It was not immediately clear what the target of Sunday's explosion was, but the damaged car was parked near a security checkpoint in the Damascus suburb of Douma. A security official at the checkpoint told the UN observers that gunmen had targeted two military buses in Douma earlier in the day, wounding more than 30 security agents.
"We obviously don't have the specifics about what happened here this morning," Mood said Sunday.
Ladsous gave a grim assessment of conditions for civilians in Douma, the scene of repeated clashes between security forces and rebels in recent months.
"[Douma] is completely paralyzed," Ladsous said. "There is still some fighting taking place … It's absolutely imperative that all parties exercise restraint and don't engage into any more fighting. It serves nothing."
Activists reported heavy shelling Sunday in the town of Soran in the central Hama province. In Damascus, opposition groups reported fighting overnight between government forces and army defectors in the district of Kfar Souseh, a hotbed of dissent against President Bashar Assad's regime. The district is a high security area, housing the Foreign Ministry and several security and intelligence agencies. It has also been the scene of frequent anti-Assad demonstrations.
The UN says more than 9,000 people have been killed in the Syrian government crackdown.