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.

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 UN ceasefire monitors left the area, may prompt more diplomatic pressure on Damascus.

Former UN 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 UN 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. - Reuters