The United Nations suspended all humanitarian convoys Tuesday in Syria after 21 people died in what UN Secretary General Ban Ki-moon called a "sickening, savage and apparently deliberate attack" on vehicles delivering aid.
The US charged that either a Russian or a Syrian airstrike hit the aid convoy late Monday, and placed most of the blame on Moscow, the main backer of Syrian President Bashar al-Assad.
Both Moscow and Damascus denied they were behind the strike. Russian Defence Ministry spokesman Igor Konashenkov said there was no sign of any bombing, based on drone footage and films from activists.
"Everything shown on the video is the direct consequence that the cargo caught fire," Konashenkov was quoted as saying by state-run news agency TASS.
The UN, Red Cross and Syrian Red Crescent - who jointly organized the aid convoy in coordination with the Syrian government - confirmed there had been an attack.
Earlier, the UN said the incident in northern Aleppo province could constitute a war crime.
"The humanitarians delivering life-saving aid were heroes ... Those who bombed them were cowards," Ban told the UN General Assembly on Tuesday in New York.
A joint statement by Red Cross-linked aid groups said 20 civilians and one Red Crescent staff member were killed and most of the aid in the convoy was destroyed.
"From what we know of yesterday's attack, there has been a flagrant violation of international humanitarian law, which is totally unacceptable," said Peter Maurer, head of the International Committee of the Red Cross (ICRC).
The UN will need to reassess the security situation in the wake of the attack, spokesman Jens Laerke said in Geneva, explaining the decision to halt aid deliveries.
The convoy bombing came just hours after the Syrian Army declared an end to a week-long truce, brokered by the United States and Russia.
The 18 trucks that were hit were part of a 31-vehicle convoy that was unloading food and medical supplies for tens of thousands of people at a warehouse of the Syrian Red Crescent (SARC), trying to take advantage of the truce.
The UN said the parties to the conflict were informed about the delivery and knew the route.
"The destination of this convoy was known to the Syrian regime and the Russian Federation, and yet these aid workers were killed in their attempt to provide relief to the Syrian people," US State Department spokesman John Kirby said.
David Swanson, a spokesman for the UN in southern Turkey, said that after the attack and the collapse of the ceasefire, millions of people in need would not receive aid.
Airstrikes have been pounding Aleppo and other areas of Syria since the ceasefire was ended Monday. Fresh fighting took place between ground forces in the north of the country, according to the Britain-based Syrian Observatory for Human Rights.
Meeting on the sidelines of the UN General Assembly, foreign ministers of the International Syria Support Group (ISSG) were still pushing for a truce.
They agreed that despite continued violence it was "imperative to pursue a nationwide cessation of hostilities" based on the agreement reached between the United States and Russia, US State Department spokesman John Kirby said.
The group, which is seeking a political solution to the conflict, said it was essential to immediately establish "unimpeded, safe and sustained flow of access for humanitarian supplies." The group of envoys from more than 20 countries plan to meet again Friday.
Kremlin spokesman Dmitry Peskov said Russia did not expect to reinstate the ceasefire, according to TASS.
US officials appeared to be holding out hope that they could negotiate a new truce deal with the Russians. The week-long truce did manage to reduce the violence in the country and allowed some aid to reach tens of thousands of people in need.
At the UN, US President Barack Obama stressed the need for a diplomatic solution in Syria that includes the delivery of humanitarian aid and a political transition.
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