The United States believes two Russian aircraft attacked an aid convoy near Aleppo in a strike that shattered a one-week truce, US officials said on Tuesday.

Russia has denied involvement.

Despite the military blame game over Monday’s attack, diplomats struggled to save the US-Russian ceasefire agreement that took effect on September 12th.

The incident, in which 18 trucks from a 31-vehicle convoy were destroyed, looked likely to deal a death blow to diplomatic efforts to halt a civil war now in its sixth year.

Two Russian Sukhoi SU-24 warplanes were in the skies above the aid convoy at the exact time it was struck late on Monday, two US officials told Reuters, citing US intelligence that led them to conclude Russia was to blame.

Russia’s foreign ministry spokeswoman denied the assertion, telling reporters at the United Nations the US administration “has no facts” to support the claim, adding: “We have nothing to do with this situation.”

Ben Rhodes, a deputy US national security adviser, said the White House held Russia responsible for what he called an “enormous humanitarian tragedy” but he did not address whether the attack was carried out by Russian aircraft.

Earlier Russia, which denied its aircraft or those of its Syrian government allies were involved, had said it believed the convoy was not struck from the air at all but had caught fire because of some incident on the ground.

The Syrian Red Crescent said the head of one of its local offices and “around 20 civilians” had been killed, although other death tolls differed. The attack prompted the United Nations to suspend all aid shipments into Syria.

Senior officials from 23 nations emerged from a one-hour meeting on Syria at a New York luxury hotel with little more than an agreement to meet again, on Friday, about how to end a conflict that has killed hundreds of thousands and driven millions from their homes.

They also differed on the chances of renewing the ceasefire.

“The ceasefire is not dead,” US Secretary of State John Kerry said after the meeting, which he hosted with Russian Foreign Minister Sergei Lavrov.

Reuters