Russian and US viewpoints also differ over the ambigous possibility of reviving a ceasefire in Syria.
The United States and Russia engaged in a war of words over the attack on a UN aid convoy in Syria's Aleppo province, which shattered a fragile truce between Bashar al Assad's regime and the conflict-ridden country's opposition forces.
US officials believe two Russian aircraft attacked the aid convoy, ending the ceasefire that was brokered by the US and Russia earlier this month.
Two Russian Sukhoi SU-24 warplanes were in the skies above the aid convoy at the exact time it was struck late on Monday, two US officials said, citing US intelligence that led them to conclude Russia was to blame.
Ben Rhodes, a deputy US national security adviser, said the White House held Russia responsible for what he called an "enormous humanitarian tragedy" but he did not address whether the attack was carried out by Russian aircraft.
Russia's foreign ministry spokeswoman denied the assertion, telling reporters at the United Nations the US administration "has no facts" to support the claim, adding: "We have nothing to do with this situation."
Russia said on Tuesday that "drone footage shows an aid convoy attacked in Syria was accompanied by a militant truck carrying a heavy mortar gun."
# # # https://t.co/cgwElHebyO pic.twitter.com/6a5gqNYqxn— (@mod_russia) September 20, 2016
Russia's Defence Ministry spokesman Igor Konashenkov issued a statement saying that neither the Russian military nor the Syrian regime had carried out any airstrikes on the humanitarian aid convoy.
"We have closely studied the video footage from where the incident took place and we did not find any signs of any ammunition having hit the convoy. There are no craters, while the vehicles have their chassis intact and they have not been severely damaged, which would have been the case from an airstrike," Konashenkov said.
"All of the video footage demonstrates that the convoy caught fire, which strangely happened almost at exactly at the same time as militants started a large scale offensive on Aleppo."
The Syrian Red Crescent said the head of one of its local offices and "around 20 civilians" had been killed, although other death tolls differed. The attack prompted the United Nations to suspend all aid shipments into Syria.
Terrible: director of #Syrias Red Crescent center round #Aleppo Omar Barakat, was killed in strike vs aid convoy: pic.twitter.com/o3Z5dswoe7— Charles Lister (@Charles_Lister) September 19, 2016
The United Nations had asked in vain for warring sides in Syria to stop attacking the aid convoy, the UN Syria and regional humanitarian coordinators, Massimo Diana and Kevin Kennedy, said in a statement.
"The UN in Syria was informed of the attacks as they unfolded. Despite our efforts and communications with parties to the conflict, further attacks continued throughout the night, hampering efforts to reach and attend to the wounded," the statement said.
An original version of the statement referred to "air strikes" rather than "attacks", but it was amended within minutes.
A UN spokesman said the error was probably due to a drafting mistake and it did not reflect a shift in the UN's view of the incident.
"The explanation is that we are not in a position to determine whether these were in fact air strikes. We are in a position to say that the convoy was attacked," said Jens Laerke, UN humanitarian spokesman in Geneva.
"We have asked for an investigation of this, and that includes how to describe the attack."
However, he rejected a theory put forward by the Russian defence ministry that the convoy caught fire.
"We have designated it as an attack and that stands," he said. "We've established it was not a fire."
The United Nations had just received permission from the Syrian regime to deliver aid to all besieged areas in the country, Laerke said.
All parties, including Russia and the United States, had been notified about the cleared convoy heading to opposition-held eastern Aleppo.
At least 18 of 31 trucks in a UN and Syrian Arab Red Crescent (SARC) convoy were hit on Monday along with an SARC warehouse. The convoy was delivering aid for 78,000 people in the hard-to-reach town of Urm al-Kubra in Aleppo province.
An air raid carried out overnight by Syrian or Russian warplanes killed four medical workers and at least nine opposition fighters near Aleppo, the Syrian Observatory for Human Rights (SOHR) monitoring group said on Wednesday.
The raid hit the town of Khan Touman southwest of Aleppo city, an area controlled by the opposition.
The medical staff killed were working for the Union of Medical Care and Relief Organizations (UOSSM), SOHR said. UOSSM confirmed in a statement that four of its staff had been killed.
Can the truce be salvaged?
As with the case of the aid convoy attack, there were different viewpoints on the possibility of reviving a ceasefire in Syria.
US Secretary of State John Kerry insisted that US-Russian attempts to broker a truce in the civil war are "not dead" and promised international talks will resume this week.
US President Barack Obama backed his top diplomat, saying a short time later that there was "no ultimate military victory to be won" and calling on nations to pursue the "hard work" of diplomacy.
Kremlin spokesman Dmitry Peskov said on Tuesday that the ceasefire could resume only if attacks against regime forces stop, according to a report on the Radio Free Europe/Radio Liberty website.
He added hope for renewal of truce was "very weak."
"The conditions are very simple," Peskov added. "The shooting needs to stop and the terrorists need to stop attacking Syrian troops."
"And of course it wouldn't hurt if our American colleagues didn't accidentally bomb the Syrians," the Kremlin spokesman also said, referring to a US-led coalition strike last week that killed dozens of regime fighters.
The attack on an aid convoy in Aleppo came just hours after the Syrian regime forces announced the end of the truce on Monday, accusing opposition forces of failing to "commit to a single element" of the US-Russia deal.
The fragile ceasefire was marred by frequent reports of violations and was already under strain after a US-led coalition strike on Saturday hit a Syrian regime post near the eastern city of Deir Ezzor.
Washington said coalition forces were targeting DAESH, which is not party to the ceasefire, and hit the regime forces by mistake.