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.

Russia has denied its aircraft or those of its Syrian regime 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.

"It was certainly not the coalition who struck from the air. It does look like an air strike," Central Command spokesman Colonel John Thomas had told reporters during a telephone briefing early Tuesday.

"The only other entities that fly in Syria are Russia and Syria," he said.

The incident could deal a powerful blow to the ceasefire, the latest attempt to halt a war in its sixth year.

The Syrian Red Crescent said the head of one of its local offices and "around 20 civilians" were killed in Monday's strike, which the Syrian Observatory for Human Rights blamed on Russian or Syrian aircraft.

Russia, which is allied with Syria's Bashar Assad, denied that either its air force or the regime armed forces was responsible. The regime's army also denied that it was involved in the attack.

The United Nations suspended aid shipments into Syria on Tuesday.

Thomas said the ceasefire in Syria was in jeopardy because of increased violence.

"This is not the vision that I think was put in place more than seven days ago, to try to get humanitarian assistance flowing into the area and try to decrease the level of violence," Thomas said.

On Saturday, Russia said that U.S. jets had killed more than 60 Assad regime soldiers.

The strike triggered a war of words between Washington and Moscow and further strained the cessation of hostilities.