WASHINGTON (Reuters) - 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.

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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.

(Reporting by Phil Stewart; Editing by James Dalgleish)