Syria's fragile ceasefire appears to have been shattered after an aid convoy was hit by air strikes that killed at least 12 people, mostly truck drivers and Red Crescent workers. It is not clear who was behind the attack. The Syrian Observatory for Human Rights monitoring group said the attacks were carried out by either Syrian or Russian aircraft, adding that there had been 35 strikes in and around Aleppo since the truce ended.

UN officials said the convoy was delivering assistance for 78,000 people in the town of Urm al-Kubra, west of the rebel-held city of Aleppo. Initial estimates indicate that about 18 of the 31 trucks in the convoy were hit, as well as the Red Crescent warehouse in the area. A local resident told Reuters by phone that the trucks were hit by about five missile strikes.

UN aid chief Stephen O'Brien said if the "callous attack" was found to be deliberate it amounted to a war crime. "Notification of the convoy ... had been provided to all parties to the conflict and the convoy was clearly marked as humanitarian," he said in a statement, calling for an immediate, independent investigation.

The UN humanitarian aid agency says it has temporarily suspended all convoys in Syria following the deadly air strike on aid trucks. Spokesman Jens Laerke of Office for the Coordination of Humanitarian Affairs (OCHA) says the temporary suspension of the aid deliveries would hold pending a review of the security situation in Syria. Laerke said the UN aid coordinator had received needed authorisations from the Syrian government in recent days to allow for aid convoys to proceed within Syria. He said it's "a very, very dark day ... for humanitarians across the world."

The Britain-based Syrian Observatory for Human Rights gave the number of casualties as 12, and the figure was confirmed by the Syrian Civil Defence, the volunteer first responder group also known as the White Helmets. Jan Egeland, humanitarian aid coordinator in the office of the UN envoy for Syria, told The Associated Press: "It is outrageous that it was hit while offloading at warehouses."

The convoy, part of an inter-agency dispatch operated by the Syrian Red Crescent, was hit in the rural western Aleppo province. The White Helmets first responder group posted images of a number of vehicles on fire and a video of the attack showed huge balls of fire in a pitch black area, as ambulances arrive on the scene.

A @SyriaCivilDef Aleppo center video with a volunteer describing in English airstrikes that hit UN/SARC convoy tonight. Convoys were loaded.

— Jake Godin (@JakeGodin) September 19, 2016

Photos from @scd_aleppo showing aftermath of airstrikes tonight in western Aleppo that reportedly hit (then double tapped) an aid convoy.

— Jake Godin (@JakeGodin) September 19, 2016

Terrible: director of #Syria’s Red Crescent center round #Aleppo Omar Barakat, was killed in strike vs aid convoy:

— Charles Lister (@Charles_Lister) September 19, 2016

The ceasefire came into effect on 12 September. Under terms of the agreement, the successful completion of seven days of calm and humanitarian aid deliveries would be followed by an ambitious second-stage plan to set up a joint US-Russian coordination centre to plan military strikes against Islamic State (Isis) and a powerful al-Qaeda-linked militant faction. However, the truce was beset by difficulties and mutual accusations of violations from the start.

Aid deliveries to the besieged eastern districts of Aleppo did not reach their destination. The UN accused the government of obstructing the deliveries, while Russian officials said rebels opened fire along the delivery roads.