Moscow has angrily denied that the Russian or Syrian air force were involved in an overnight attack on a humanitarian convoy.
The Russian Foreign Ministry said: "We responsibly say that neither the Russian nor the Syrian air force conducted any strikes on the UN aid convoy on the south-western outskirts of Aleppo."
The ministry described 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 US-led coalition.
Syrian activists and paramedics have said the air strikes on the convoy were conducted by Russian or Syrian aircraft.
The attack on Monday night killed 20 civilians and prompted the UN to suspend all aid convoys in Syria.
Meanwhile, UN Secretary-General Ban Ki-moon denounced the "sickening, savage and apparently deliberate attack" on the Arab Red Crescent convoy, calling the bombers "cowards".
Mr Ban told world leaders at the opening of the UN General Assembly's ministerial meeting that "just when we think it cannot get any worse, the bar of depravity sinks lower".
The Red Crescent said about 20 civilians were killed. Mr Ban called those delivering aid "heroes".
He said the UN was forced to suspend aid convoys "because of this outrage".
Mr Ban blamed the Syrian government for most deaths and said it "continues to barrel bomb neighbourhoods and systematically torture thousands of detainees".
He added countries "that keep feeding the war machine also have blood on their hands", and accused unnamed governments attending the UN meeting of ignoring, facilitating, funding, planning and carrying out "atrocities" against civilians on all sides.
The attack plunged Syria's US-Russia-brokered ceasefire further into doubt. The Syrian military, just hours earlier, had declared the week-long truce had failed.
The United States said it was prepared to extend the truce deal and Russia - after blaming rebels for the violations - suggested it could still be salvaged.
It was not clear who was behind the attack late on Monday.
Both Syrian and Russian aircraft operate over the province, while the US-led coalition targets the Islamic State group in other parts of the country.
In Geneva, Jens Laerke, a spokesman for the UN Office for the Coordination of Humanitarian Affairs, said that all aid deliveries had been halted pending a review of the security situation in Syria in the aftermath of the attacks.
Mr Laerke called it "a very, very dark day... for humanitarians across the world".
A member of the Syrian Civil Defence - a group of volunteer first responders also known as the White Helmets - criticised the UN humanitarian aid agency for suspending the convoys.
Ibrahim Alhaj said that Syrian civilians will pay the price for the decision - and that the UN should have condemned the attacks on the convoy rather than suspending aid.
The convoy was part of a routine interagency dispatch operated by the Syrian Red Crescent.
UN officials said it was delivering assistance for 78,000 people in the town of Uram al-Kubra, west of the city of Aleppo. Initial estimates indicate that about 18 of the 31 trucks in the convoy were hit, as well as the Red Crescent warehouse in the area.
World Health OrganiSation spokesman Christian Lindmeier said the convoy was expected to carry medicines, emergency health kits, trauma kits, burn kits and IV fluids.
"All this was supposed to have been on that convoy, but I do not have a breakdown of what got destroyed and what did not get destroyed," he said.
UN humanitarian chief Stephen O'Brien called on "all parties to the conflict, once again, to take all necessary measures to protect humanitarian actors, civilians, and civilian infrastructure as required by international humanitarian law".
Russian, Syrian Air Forces did not strike UN aid convoy in Aleppo - Russian MoD pic.twitter.com/1ZvlozA3pu— RT (@RT_com) September 20, 2016