The US believes Russian warplanes were responsible for an attack on a UN aid convoy operated by the Syrian Arab Red Crescent, officials have told CNN.

At least a dozen aid workers are thought to have been killed by the air strike, which occurred near Aleppo on Monday, hours after the breakdown of a ceasefire brokered by the US and Russia.

UN Secretary-General Ban Ki-Moon has described the incident as "sickening, savage and apparently deliberate." Mr Ban said the UN had suspended its aid convoys following the attack. Speaking to world leaders at the UN General Assembly in New York, he said: "The humanitarians delivering life-saving aid were heroes. Those who bombed them were cowards.“

The bombing was initially blamed on either Russia or Syria's Assad regime. Both have denied that their planes carried out the strike, but the US has reportedly reached the "preliminary conclusion" that two Russian Su-24 jets were responsible, based on the timing and location of the attack. The strike hit at least 18 of the 31 trucks in the convoy, which was bringing food relief from UN stores to the town of Urm al-Kubra, west of Aleppo, in an area controlled by rebel groups.

The attack came amid a resumption of hostilities following the fragile, week-long ceasefire, which it was hoped would mark the beginning of the end of Syria's bloody five-year civil war.

The Russian Defence Ministry said its intelligence services had studied drone footage from the incident, and did not see evidence of munitions hitting the convoy. The damage inficted on the trucks was "the direct result of the cargo catching fire, which mysteriously began at the same time as a large scale rebel attack on Aleppo,” Russian defence ministry spokesman Igor Konashenkov said.

A section of the Russian footage was also released, which Mr Konashenkov said "clearly shows" rebels driving a pick-up truck and a mortar, using the convoy as cover.

The US had already made it clear on Monday night that it held Moscow responsible for the deaths, regardless of whose planes carried out the strike.

“We don’t know at this point whether it was the Russians or the regime. In either case, the Russians have the responsibility certainly to... refrain from taking such action themselves, but they also have the responsibility to keep the regime from doing it,” a US State Department statement said.

US Secretary of State John Kerry – who personally negotiated the truce during months of intensive diplomacy with Russia – spoke about the crumbling ceasefire after emerging alongside Russian Foreign Minister Sergei Lavrov from a meeting of foreign ministers of 20 countries, gathered to discuss Syria in New York on Tuesday.