US defence officials now believe that Russian planes dropped the bombs that destroyed a UN aid convoy that killed at least 20 people, the Guardian has learned.

The claim of direct Russian involvement in the bombing, if confirmed, would have far-reaching consequences. Earlier on Tuesday, Ban Ki-moon used his farewell address to the UN general assembly to denounce it as a “sickening, savage and apparently deliberate attack”, describing the bombers at “cowards”, and UN officials have said it is a potential war crime.

The outgoing secretary general told world leaders in New York that the UN had been forced to suspend aid convoys in Syria because of Monday’s attack on Syrian Red Crescent trucks that were carrying UN food supplies to a rural area west of Aleppo city.

Victims of the attack included the local director of the Syrian Arab Red Crescent, Omar Barakat. Ban hailed the dead aid workers as heroes and said “those who bombed them were cowards” before calling for accountability for crimes committed in the war. “Just when you think it cannot get any worse, the bar of depravity sinks lower,” he said.

Aid officials said the convoy was hit from the air while food and medicine were being unloaded at a warehouse in opposition-controlled Urem al-Kubra.

Reuters news agency quoted two US officials as saying two Russian Sukhoi SU-24 warplanes were in the sky above the aid convoy at the precise time it was struck.

The White House and state department said they could not confirm the allegations, while the Russian foreign ministry rejected them with “resentment and indignation”.

Previously, US officials had said that they would hold Moscow responsible for the attack, even if it was carried out by Syrian aircraft, as Russia had taken responsibility for the regime’s compliance with the ceasefire as part of the 9 September agreement.

But Moscow has not conceded that the convoy was hit by an airstrike, claiming instead that the 18 lorries had “caught fire”. In a separate statement on Tuesday, the country’s defence ministry said that the aid convoy had been accompanied by a militants’ pickup truck armed with a heavy mortar , Russian news agencies reported.

The US officials said there was no doubt the convoy was destroyed in an airstrike and that western coalition forces had no role in it.

“There are only three parties that fly in Syria: the coalition, the Russians and the Syrian regime. It was not the coalition. We don’t fly over Aleppo. We have no reason to. We strike only I sis, and I sis is not there. We would leave it to the Russians and the Syrian regime to explain their actions,” said Capt Jeff Davis, a Pentagon spokesman.

In a meeting with John Kerry, Sergey Lavrov, the Russian foreign minister, admitted that the Russian military had been monitoring the convoy – apparent drone surveillance footage of its progress had been live-streamed on a defence ministry website . But he claimed the Russians had “lost track of it when it entered rebel territory ”, according to diplomatic sources. Moscow had launched an investigation, Lavrov told the other foreign ministers.

Later on Tuesday, however, the Russian foreign ministry put out an angry denunciation of allegations against Moscow and Damascus.

“We are considering, with resentment and indignation, attempts by some foreign curators of rebel units and terrorists in Syria to put the blame for the incident on the Russian and Syrian air forces who allegedly bombarded a relief convoy,” the statement said, according to the Tass news agency.

Despite the outrage caused by the attack, and the continued bombing of rebel-held areas of Aleppo, the US and Russia have refused to declare the Syrian ceasefire dead.

Lavrov’s meeting with the US secretary of state was the first since the Syrian military declared itself no longer bound by the ceasefire the two politicians negotiated , and resumed its air campaign against eastern Aleppo and other rebel-held areas on Monday.

Lavrov and Kerry talked in a central New York hotel for about half an hour on Tuesday morning before walking, still in deep discussion, into a broader session with other foreign ministers from the security council, Europe and the Middle East who make up the International Syria Support Group (ISSG). Kerry emerged from the group’s meeting insisting: “The cease fire is not dead .”

“We are going to continue to work. We are going to meet again Friday on some specific steps,” the secretary of state said.

Boris Johnson, the UK foreign secretary, said: “Quite frankly, the Kerry-Lavrov process is the only show in town and we have to get that show back on the road.”

The UN later retreated from the claim that the convoy had been targeted in an airstrike. “We are not in a position to determine whether these were in fact airstrikes. We are in a position to say that the convoy was attacked,” its humanitarian spokesman, Jens Laerke, said.