Two Russian Sukhoi SU-24 warplanes were in the skies above an aid convoy in Syria at the precise time it was struck on Monday, two U.S. officials told Reuters on Tuesday, citing U.S. intelligence that has led them to conclude Russia was to blame.

UN Special Envoy for Syria, Staffan de Mistura, said it was an “outrage”.

“The convoy was the outcome of a long process of permission and preparations to assist isolated civilians,” he said in a statement emailed to Reuters.

The Syrian Red Crescent said the convoy had been making a routine delivery from Aleppo to rural rebel-held areas. Images posted online showed what appeared to be aid lorries engulfed by flames.

UN aid convoy hit by airstrike near Aleppo, killed at least 12 people

Russia has denied its aircraft or those of its Syrian government allies were involved in the incident, in which 18 trucks from a 31-vehicle convoy were destroyed. The strike appeared to deal a fatal blow to Syria’s fragile week-old ceasefire.