Syria has failed to comply with a pledge to withdraw weapons from cities, and citizens who meet U.N. truce monitors may have been killed, international mediator Kofi Annan will tell the Security Council on Tuesday, his spokesman said.

As violence flared in the Syrian capital Damascus, spokesman Ahmad Fawzi said Annan would ask for a stronger presence of monitors to watch over the country's ragged ceasefire.

Satellite imagery showed Syrian forces had not withdrawn heavy weapons from urban centres and returned to their barracks, as they are required to do under a plan drawn up by Annan, said Fawzi.

This is unacceptable, and Joint Special Envoy Kofi Annan will be saying this to the Security Council today when he addresses them in closed session, Fawzi added in remarks on U.N. Television.

Annan had also received credible reports that after monitors left a town, people who met them were approached by Syrian soldiers or even worse, perhaps killed, said Fawzi.

Observers from the fledgling United Nations mission visited on Tuesday 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.

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

They (Syrian authorities) are claiming that this has happened. Satellite imagery, however, and credible reports show that this has not fully happened, Fawzi said.

Three Syrian military officers were killed in Damascus on Tuesday, state media and opposition groups said, and at least three people were wounded in a car bomb blast in the capital in further blows to the accord.

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 neighbourhood.

Damascus residents said the explosion in a pickup truck directly outside an Iranian cultural centre, in a popular shopping district, was loud but caused 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.

Damascus says 2,600 of its security 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 guerrilla attacks.

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

CEASEFIRE INCOMPLETE

U.N. officials said 11 members of a planned 300-strong monitoring mission were now in Syria to track the stuttering progress of the April 12 truce engineered by Annan.

Anti-government groups say even the minimal U.N. presence has led to some decline in the daily death toll although they accuse the army of simply parking tanks out of sight and resuming operations the moment monitors' backs are turned.

Two observers were now staying permanently in Hama, a U.N. official in Geneva said, after activists said 31 people were killed on Monday in shelling and shooting in the city, a hub of the anti-Assad uprising, immediately after a visit by a monitoring team.

Two are stationed in Hama now. They went back to Hama today because of reports of killing of a significant number of people with automatic weapons after they had been there, Annan spokesman Ahmad Fawzi said.

An activist in Douma, a town east of Damascus that received a U.N. monitoring visit on Monday, said there were tanks on the streets and two buildings had been hit by shells.

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 Muammar 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.

VAIN HOPE FOR U.N. MISSION

U.N. Secretary-General Ban Ki-moon will brief the Security Council every 15 days about developments in Syria and submit proposals as needed for adjusting the mandate of the observer mission, to be called UNSMIS.

Monday's shelling in Hama has deepened scepticism that the monitoring mission - even when it reaches full size - will effect a lasting reduction in violence.

Tunisian President Moncef al-Marzouki was the most candid, telling the al-Hayat regional newspaper it was doomed to fail.

I do not expect it to succeed, because the number of observers is very small. Three hundred people cannot do anything, he said.

The grinding conflict has crippled Syria's oil- and tourism-driven economy, leaving at least a million people in need of humanitarian aid, according to a joint U.N.-Syrian assessment mission reached last month.

As with foreign journalists, U.N. aid agencies have been largely barred from Syria, although the U.N. World Food Programme said it aimed to deliver aid to 500,000 people in the coming weeks, roughly double the number it expects to reach this month.

(Additional reporting by Dominic Evans, Louis Charbonneau and Michelle Nichols at the United Nations, Stephanie Nebehay in Geneva and John Irish in Paris; Writing by Ed Cropley; Editing by Andrew Heavens)