Observers from the fledgling United Nations mission visited the central province of Homs, hotbed of a 13-month-old uprising against President Bashar al-Assad, as part of efforts to silence the guns 12 days after the accord was struck.

SANA, the state news agency, said an "armed terrorist group" shot dead two army officers near Damascus, while the UK-based Syrian Observatory for Human Rights said a third was killed in the capital's Barzeh neighborhood.

Damascus residents described the explosion in a pickup truck directly outside an Iranian cultural centre, in a popular shopping district, as extremely loud but causing limited damage.

Windows in nearby shops were not shattered and there were no signs of damage to the centre, run by Assad's powerful regional ally, Tehran. Shopkeepers said four people were injured, including a taxi driver.

The pro-Assad Ikhbaria television channel blamed the blast on "armed terrorists" - shorthand for the rebels who have been fighting to overthrow Assad, inspired by Arab Spring uprisings against autocratic rulers in North Africa and the Middle East.

The United Nations says security forces have killed at least 9,000 people in the conflict, while Damascus says 2,600 of its personnel have died at the hands of insurgents who have seized control of pockets of towns and cities across the country of 23 million and who continue to launch daily guerrilla attacks.

SANA said on Tuesday customs officials on the Syria-Lebanon border had seized a car stuffed with ammunition and weapons, including three machineguns and a rocket-propelled grenade launcher.

CEASEFIRE "INCOMPLETE"

A small group of unarmed U.N. observers has been in Syria for just over a week tracking the stuttering progress of the April 12 truce engineered by former U.N. Secretary General Kofi Annan.

Under its terms, both sides are supposed to adhere to a ceasefire while the army withdraws tanks and heavy weapons from population centers - requirements that the U.N. and France have made clear are not being heeded.

France said it still supported Annan's peace plan but could not do so forever without changes on the ground, most notably in the deployment of pro-Assad forces.

"The regime must not get it wrong this time," Foreign Ministry spokesman Bernard Valero said. "It cannot continue to mislead the international community for much longer. When the time comes, we will have to take the necessary measures required if the situation on the ground continues."

For all the rhetoric, France and other Western powers have few tools at their disposal to get at Assad, who succeeded his long-ruling father Hafez in 2000 and who has brushed aside all calls to hand over power.

Military intervention similar to the air campaign in Colonel Gaddafi's Libya could draw in powerful Assad allies such as Iran and Hezbollah militants, and Russia and China are opposed to the U.N. sanctions that Washington and Europe are calling for.

The U.N. observer mission is meant to grow to 300 unarmed personnel although the modest presence already on the ground has led to some decline in the daily death toll, activists say.

However, they accuse Assad's army of simply parking tanks out of sight on farms on the outskirts of towns and cities and resuming operations the moment the monitors' backs are turned.

Activists said that 31 people were killed on Monday in shelling and shooting in the central city of Hama, a hub of the revolt, the day after a brief visit by a U.N. team.

"It began in the morning with tanks and artillery. There were houses burning," a local opposition activist who identified himself as Mousab told Reuters in neighboring Lebanon.

Video that activists said was shot in Hama showed 10 bodies wrapped in shrouds being loaded into a mass grave. Another 24 people were killed in violence elsewhere on Monday, according to anti-Assad groups. (Reuters)