"All of our information indicates clearly that this was an airstrike," White House Deputy National Security Advisor Ben Rhodes said late Tuesday.

"That means that there only could have been two entities responsible - the Syrian regime or the Russian government," he said, adding that the US holds the Russian government responsible for airstrikes in the area, pointing to a Russian pledge under a ceasefire agreement.

Monday's incident destroyed 18 of the 31 vehicles in the motorcade, killing 21 people. The trucks were unloading food and medical supplies for tens of thousands of people at a warehouse of the Syrian Red Crescent at the time of the attack.

The incident shattered a fragile one-week truce and led to the United Nations suspending all humanitarian convoys in Syria.

According to US officials speaking to the Reuters news agency, citing US intelligence, two Russian Sukhoi SU-24 warplanes were in the skies above the aid convoy at the exact time it was struck.

But both Moscow and Damascus denied any involvement in the strike. Russian Defense Ministry spokesman Igor Konashenkov said there was no sign of any bombing, based on drone footage and films from activists. The Kremlin said it believed the convoy had caught fire because of some incident on the ground.

The UN, Red Cross and Syrian Red Crescent jointly organized the aid convoy in coordination with the Syrian government. The Syrian Red Crescent said the head of one of its local offices and "around 20 civilians" had been killed.

The United Nations believes the incident could constitute a war crime.

"The humanitarians delivering life-saving aid were heroes ... Those who bombed them were cowards," UN Secretary-General Ban Ki-moon told the UN General Assembly on Tuesday.

The convoy was hit just hours after the Syrian Army declared an end to a week-long truce, brokered by the US and Russia.

The UN Security Council is set to hold a high-level meeting on Syria on Wednesday.

(DW)