By Oliver Holmes
BEIRUT, April 25 (Reuters) - Syrian security forces shot dead four civilians on a bus in the northern province of Idlib on Wednesday, dissidents said, as international pressure built on Damascus to honour ceasefire pledges to order soldiers and tanks back to barracks.
In the latest violence to undermine the flaky 13-day-old truce, the British-based Syrian Observatory for Human Rights said the four were killed at a checkpoint on the main road from Aleppo to the capital.
Internet video that activists said was shot soon after the incident showed the bodies of two women and a wounded man lying on stretchers.
There was no mention of the shooting in Syria's rigidly controlled media or comment from the authorities in Damascus, which has barred most foreign journalists during 13 months of an uprising against President Bashar al-Assad.
The bus attack, occurring two days after 31 people were killed in Hama city immediately after U.N. ceasefire monitors left the area, may prompt more diplomatic pressure on Damascus.
Former U.N. Secretary-General and ceasefire broker Kofi Annan told the Security Council on Tuesday that Syria had failed to withdraw weapons from population centres in violation of the terms of the April 12 truce.
"The situation in Syria continues to be unacceptable," he said, according to a transcript. "The Syria authorities must implement their commitments in full and a cessation of violation in all its forms must be respected by all parties."
He also noted reports that security forces were targeting people who had met members of the fledgling U.N. monitoring mission, which is meant to end a conflict in which the United Nations says at least 9,000 people have been killed.
"If confirmed, this is totally unacceptable and reprehensible," Annan said.
Damascus says 2,600 of its security personnel have been killed by the anti-Assad armed groups that operate in parts of the country of 23 million.
Speaking to the 15-nation Security Council, Annan stressed the need to get "eyes and ears on the ground", although so far there are only 15 unarmed monitors in Syria out of a planned final team of 300 to be deployed under the acronym UNSMIS.
Reasons for the slow deployment are not clear.
Activists say even the minimal UNSMIS presence has led to a drop in the daily death toll, but U.N. peacekeeping chief Herve Ladsous said it would take a month to deploy the first 100 monitors - a timeline that drew derision from many Syrians.
"It takes them a month to arrive? Are they coming on horses?" asked a resident of Homs, a city which has endured sustained army shelling. He declined to give his real name.
Amateur videographers have filmed the small teams of monitors travelling in their distinctive blue U.N. helmets and bullet-proof vests meeting rebels and residents of shelled neighbourhoods in towns and cities across the country.
In a display of Syrian black humour, some have also mocked the monitors, appearing on video in spoof blue uniforms and with blacked-out glasses and tissue paper stuffed into their ears - pretending neither to see nor hear anything untoward.
"After one month we will have maybe 1,000 or 2,000 people killed - it's ridiculous. How can the international community watch without moving quickly?" asked Mousab al-Hamadi, an opposition resident in Hama province, a hotbed of the revolt.
Annan said Syrian Foreign Minister Walid al-Moualem had written to him saying that "the withdrawal of massed troops and heavy weapons from in and around population centres is now complete and military operations have ceased".
However, Annan's team also cited satellite imagery as evidence that tanks are lurking out of sight on the outskirts of cities, and even Syria's ally Russia voiced concern.
Russia's U.N. Ambassador, Vitaly Churkin, said it would be worrying if Damascus had failed to withdraw troops and weapons.
"If this is the case, if the promise in the letter has not really been carried out, that would mean it is a breach of the promise they made on Saturday," Churkin said. "I'm certainly going to bring it to the attention of Moscow."
Throughout the conflict, Russia has been one of Syria's few friends, providing protection at the United Nations from any Security Council measures against Assad's government.
France said it still supported Annan's peace plan but could not do so forever unless Syria implemented it fully.
"The regime must not get it wrong this time," said Foreign Ministry spokesman Bernard Valero. "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 to dislodge Assad, who succeeded his long-ruling father Hafez al-Assad in 2000 and who has brushed aside all calls to hand over power.
They are particularly wary of military intervention similar to NATO's Libya air campaign that helped topple Muammar Gaddafi for fear it could draw in powerful Assad allies such as Iran and Hezbollah militants and further destabilise the Middle East. (Writing by Ed Cropley; Editing by Alistair Lyon)