A U.S. official tells CBS News “preliminary indications” are that it was a Russian aircraft that struck an aid convoy attempting to deliver humanitarian supplies to a rebel-held area, killing some 20 civilians.

The U.N. humanitarian aid agency said it has temporarily suspended all convoys in Syria following the attack on the convoy, plunging the already grim five-year-old Syrian war deeper into uncertainty.

The Russians have angrily denied the allegations that they or the Assad regime were responsible. The claims of Russian or Syrian fault began right after the strike when opposition and aid groups, including the Syrian Arab Red Crescent, said it had to have been an aircraft that hit them.

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At the same time as it offered denials, the Russian Defense Ministry also released drone footage it says shows that the aid convoy was accompanied by pickup truck of militants with a heavy mortar gun. It’s not clear why Russia released the video or if they were suggesting a possible justification for an attack. The footage (posted below) has not been independently verified.

Russia’s Defense Ministry said no Russian or Syrian aircraft had carried out the strikes, and claimed only rebel factions knew the precise whereabouts of the aid convoy. According to the Interfax news agency, the Defense Ministry later said it had seen no evidence of a strike of any kind on the convoy, suggesting the approximately 20 trucks and the warehouse that were struck in an apparently prolonged attack simply caught on fire.

Various Russian officials have also claimed the convoy could have been struck by artillery. Reuters reports the United Nations rowed back on Tuesday from describing an attack on an aid convoy in Syria as airstrikes, saying it did not have conclusive evidence about what had happened.

Igor Konashenkov, a spokesman for Russia’s Defense Ministry, said in a statement obtained by Reuters: “We have studied video footage from the scene from so-called ‘activists’ in detail and did not find any evidence that the convoy had been struck by ordnance. There are no craters and the exterior of the vehicles do not have the kind of damage consistent with blasts caused by bombs dropped from the air.”

Hussein Badawi, the local head of the Syrian Civil Defense Force, first responders known commonly as the “White Helmets,” told the Associated Press on Tuesday that his crew observed at least 20 missiles, some from aircraft and some appearing to be ground-launched, slam into the convoy over a two-hour period.

The Russian Foreign Ministry on Tuesday said that “we responsibly say that neither the Russian nor the Syrian air force conducted any strikes on the U.N. aid convoy on the southwestern outskirts of Aleppo.”

It describes claims that Russian or Syrian aircraft were involved as “hasty and unfounded,” adding the allegations could be aimed at distracting attention from an earlier strike on Syrian army positions by the U.S.-led coalition.

A CENTCOM briefer, Col. John Thomas, told CBS News correspondent David Martin that the military has put all preparations for a joint operations center with the Russians “on hold” and “is not anticipating any great progress any time soon,” as the attack on the convoy and the collapse of a U.S.-Russia brokered cease-fire unfolds​.

He said “we do think it was an air strike” that hit the Red Cross convoy and “it wasn’t us.”

He also provided some additional detail on the U.S.-led coalition’s bombing of the Syrian army position. He said the strike lasted between 30 and 50 minutes. It was scheduled to last for more than an hour. The command center in Qatar received two calls from the Russians. The first simply asked to speak with the officer who was designated to talk with the Russians over the phone line set up to deconflict aircraft. It took some time “to fetch” that officer, a colonel, and as soon as he got on the line the Russian said they were striking a Syrian position and the strike was called off. He also said that the location had been under observation for two days and the strike was approved by a general officer after convening an advisory committee – in other words, the standard operating procedure.