UN Secretary-General Ban Ki-moon has announced that he is setting up a board of inquiry to investigate the bombing of an aid convoy in Syria last week that killed 18 people.
US officials have said that Russian planes carried out the air strikes on 19 September that hit the 31-truck convoy bringing aid to a town west of the besieged city of Aleppo.
Moscow has denied the accusation and the Russian military is carrying out its own investigation of the bombing that destroyed 18 trucks and damaged a warehouse.
The internal UN panel "will ascertain the facts of the incident" and report to Mr Ban, who will "decide what further steps to take," said a UN statement.
Mr Ban urged all sides to fully cooperate with the probe.
The United Nations has warned that the attack on the aid convoy could amount to a war crime.
UN aid chief Stephen O'Brien told the Security Council yesterday that the area around the convoy that was "clearly marked UN and Syrian Arab Red Crescent" became a "killing zone" during two hours of heavy bombing.
The strikes on the convoy in Urum al-Kubra claimed the life of the local head of the Syrian Arab Red Crescent, Omar Barakat, as well as 12 volunteers and five drivers.
Another 15 drivers were wounded, many civilians were killed and wounded, and the warehouse as well as a nearby medical clinic severely damaged, Mr O'Brien said.
The aid chief stressed that all sides on the ground were informed of the convoy's movement in line with UN procedures for all deliveries of humanitarian assistance.
"We do not yet have all the details," O'Brien said. "However, it is not too early to make clear the consequences of this shameful attack."
"If the attackers knew of the humanitarian convoy and intentionally directed an attack against it, they committed a war crime."
The bombing of the aid convoy took place just hours after the Syrian military announced the end of a week-long ceasefire negotiated by Russia and the United States.
Russia and the United States have traded blame for the collapse of the deal that would have marked the first step in a new effort to end the war that has killed 300,000 people since 2011.
Mr Obama and Ms Merkel urged parties on the ground to "continue respecting the ceasefire, begin the disengagement of forces along the line of contact as quickly as possible and provide international monitors unfettered access to the entire conflict zone".
"The president and chancellor strongly condemned the barbarous Russian and Syrian regime airstrikes against eastern Aleppo," the White House said.
9,300 reported dead in year of Russian air strikes
Around 3,800 civilians have been killed in one year of Russian air strikes in Syria in support of President Bashar al-Assad, a monitoring group has said.
They are among more than 9,300 people who have died in the raids since 30 September 2015, the Syrian Observatory for Human Rights said.
The toll also includes more than 5,500 fighters from the so-called Islamic State group and various rebel factions, the British-based monitor said.
At least 20,000 civilians were wounded in the Russian raids, it said.
The Observatory, which relies on a network of sources inside Syria for its information, says it determines what planes carried out raids according to their type, location, flight patterns and the munitions involved.
Observatory director Rami Abdel Rahman said the death toll from Russia strikes could be even higher given the number of people killed by unidentified warplanes.
US President Barack Obama and German Chancellor Angela Merkel earlier condemned what they called "barbarous" Russian and Syrian regime airstrikes on Aleppo during a phone call last night, according to a White House spokesperson.
The Syrian government and its ally Russia "bear special responsibility for ending the fighting in Syria," the two leaders agreed.
Mr Obama and Ms Merkel strongly condemned the strikes in eastern Aleppo, an area they said is "populated with hundreds of thousands of civilians, half of whom are children".
Two of the largest hospitals in the city's east were bombed on Wednesday in what Mr Ban described as a war crime.
Air strikes pounded Aleppo province yesterday while at least 11 civilians, including seven children, died during attacks on the city of Idlib, nearby Jarjanaz and central Hama province, the Syrian Observatory for Human Rights said.
Meanwhile, Director of Médecins Sans Frontières in Ireland Sam Taylor has said the numbers of dead in the Syrian war is "mind boggling".
Speaking on RTÉ's News at One, Mr Taylor described the city of Aleppo as a "killbox" where people are trapped and being bombed continuously.
People are dying in increasing numbers and the bombing is incessant, he added.
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