Russia Wednesday charged a U.S. drone was responsible for bombing a 31-truck U.N. aid convoy loaded with food and medicine in Syria, one day after the U.S. blamed Moscow and Tehran for the two-hour attack that killed 20 aid workers near Aleppo.

The Russian Defense Ministry said a U.S. Predator drone was in the area just minutes before the Monday incident. But U.S. officials said two Russian Sukhoi SU-24 warplanes were in the sky at the time of the strike.

"Only the drone's owners know what it was doing there at the necessary moment and what tasks it was carrying out," Reuters quoted Maj. Gen. Igor Konashenkov as saying in a statement. He also denied any Russian involvement, saying no Russian planes were in the area at the time.

U.S. Secretary of State John Kerry Tuesday told the U.N. Security Council session on Syria Russia appears to be operating “in a parallel universe.” Kerry admitted coalition forces accidentally hit a Syrian military camp Saturday, killing 62 Syrian soldiers, calling it a terrible accident. But, he said, the U.S. admitted its mistake and the Defense Department apologized.

“But I got to tell you, people running around with guns on the ground, from the air, is a very different thing from trucks in a convoy with Big U.N. markings all over them,” Kerry said.

Kerry noted Russia initially said the attack on the convoy was necessary as part of an alleged offensive against the al-Nusra Front. Then the Russian ambassador said Syrian forces were targeting Khan Tuman, a village in northern Syria. Then the Russian Defense Ministry said the aid convoy was accompanied by militants in a pickup truck with a mortar. Witnesses, however, said the sky was filled with fighter jets during the attack, Kerry noted.

"There only could have been two entities responsible, either the Syrian regime or the Russian government," deputy U.S. national security adviser Ben Rhodes said Tuesday. "In any event, we hold the Russian government responsible for airstrikes in this space."

Kerry called for all aircraft around aid routes be grounded so “humanitarian assistance [can] flow unimpeded.”