An attack on aid trucks and a warehouse in Syria has killed around 20 civilians, leading the UN to suspend all humanitarian convoys in the war-ravaged country.
Here is what we know about the attack that has also sparked a war of words between Washington and Moscow:
- What happened? -
Trucks carrying food and medical equipment from the United Nations and other agencies were unloading aid into a warehouse in the town of Orum al-Kubra in Aleppo province on Monday.
A spokesman for the UN's humanitarian agency (OCHA) in Geneva, Jens Laerke, said "every single partner or party to the conflict" was informed in advance about the delivery.
According to the Red Cross the first report that the trucks and warehouse had been hit came at 8:15 pm local time (1715 GMT), several hours after the Syrian army had declared an end to a week-long partial ceasefire.
Among the dead were a Syrian Arab Red Crescent (SARC) staff member Omar Barakat and volunteers.
Pictures of the convoy published by SARC on Twitter showed that several of the trucks were draped with blue flags from UNICEF, the UN's agency for children.
The raids destroyed at least 18 of 31 vehicles, the warehouse and much of the aid, the Red Cross said.
Inside the trucks was sanitation and nutritional supplies for 50,000 people, nine tonnes of medical aid, including antibiotics and surgical materials, according to UNICEF and the World Health Organization.
- Who was responsible? -
The Syrian Observatory for Human Rights, a monitor group, reported Monday night that an airstrike had hit the trucks, with a spokesman for the Red Cross in Geneva also referring to an air strike on Tuesday morning.
Although not confirmed, this meant that either Russian or Syrian aircraft were strongly suspected of being responsible as neither opposition groups nor jihadist groups have air power.
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Giving further details on Tuesday, an unnamed US official told AFP that Washington believed a Russian plane was responsible. Two Russian SU-24 warplanes were operating in the area, he said.
But both Syria and Russia have denied responsibility for the attack, with Moscow furious at the "unsubstantiated, hasty accusations" from the US.
The Russian military, which is investigating the incident, said that footage from activists at the scene showed damage to the vehicles that did not appear to come from an air strike or other munitions.
Later on Tuesday, the Red Cross changed the language in a press statement and referred to an "attack" on the convoy, not an air strike specifically.
The main Syrian opposition group said Tuesday it was certain that bombing from either Syrian or Russian aviation was responsible.
"No one else has aircraft in that area," said leader Riad Hijab in New York.
Hijab said a member of his High Negotiations Committee (HNC) was escorting the convoy had provided him with photographs of the attack that he described as "very precise."
The head of the International Committee of the Red Cross, Peter Maurer, said an investigation was needed into a "flagrant violation of international humanitarian law."
- What is the fallout? -
Laerke said the UN had suspended convoy movements "as an immediate security measure" and that future aid deliveries would depend on a "further assessment of the security situation."
The Red Cross has so far not followed the UN's lead in pausing deliveries, spokesman Benoit Carpentier told AFP.
Stalled aid deliveries have emerged as one of the most devastating consequences of the Syrian conflict, with the UN, Red Cross and others repeatedly demanding unhindered access to civilians in desperate need of life-saving supplies.
Even before the strikes, some 40 UN trucks carrying relief supplies destined for rebel-held east Aleppo had remained stuck in a customs zone between the Turkish and Syrian borders since early last week.