The United States has blamed Russia for an overnight attack on an aid convoy that killed 20 civilians, as the United Nations announced it was suspending overland deliveries in Syria, jeopardising food and medical security for millions of besieged and hard-to-reach civilians.
Confusion continued about who struck the convoy, but the White House insisted it was either Russia or Syria.
White House deputy national security adviser Ben Rhodes said either way the US held Russia responsible because it was Russia's job under the week-old ceasefire to prevent Syria's air force from striking in areas where humanitarian aid was being transported.
"All of our information indicates clearly that this was an airstrike," he said, rejecting the claim by Russia's Defence Ministry that a cargo fire caused the damage. Both Russia and Syria have denied carrying out the bombing.
Within one minute of the strike, the US tracked a Russian-made Su-24 directly over the region of the attack, officials said. Even that revelation failed to definitively implicate Russia because both the Russian and Syrian air forces fly the Su-24, although the US officials said there were strong indications that the jet was flown by the Russian military.
Witnesses described Monday's attack on a Syrian Arab Red Crescent warehouse and convoy in the rebel-held town of Uram al-Kubra in Aleppo province as prolonged and intense, saying the aerial bombardment continued as rescue workers rushed to pull the wounded from the flaming wreckage and rubble.
The convoy was part of a routine dispatch operated by the Syrian Red Crescent, which UN officials said was delivering assistance to 78,000 people in Uram al-Kubra, west of Aleppo city. It was carrying food, medicines, emergency health kits, IV fluids, and other essentials supplied by the UN and the World Health Organisation.
Paramedic and media activist Mohammad Rasoul, who was among the first to arrive at the scene, said more than 100 tons of food, medicine, and baby formula had gone up in flames. He said 18 of the convoy's 31 lorries were destroyed.
The attack "erased the convoy from the face of the earth", he said.
"I've never seen anything like this attack. If this had been a military position it wouldn't have been targeted with such intensity."
He said the attack began around 20 minutes after sunset on Monday and continued for two hours.
Pointing to the fact that Syria's rebels do not possess an air force, the White House said process of elimination indicated that either Syria's military or Russia's launched the attack.
Both Syrian and Russian aircraft operate over the province, while the U.S.-led coalition targets the Islamic State group in other parts of the country.
At the same time the attack took place on Uram al-Kubra, presumed Syrian or Russian jets launched a wave of attacks in and around the nearby city of Aleppo, minutes after Syria's military announced a week-long ceasefire had expired.
A cargo fire would not explain the footage filmed by rescuers of torn flesh being picked from the wreckage, or the witness accounts of a sustained, two-hour barrage of missiles, rockets, and barrel bombs - crude, unguided weapons that the Syrian government drops from helicopters.
Hussein Badawi, the head of the town's Syrian Civil Defence search and rescue group - also known as the White Helmets - said that on the night of the attack he heard the sounds of overhead ballistic missiles, helicopters and fighter jets. He and other witnesses reported seeing a reconnaissance aircraft observing the convoy before the attack.
"There were reconnaissance flights before the air strikes," said Mr Badawi. "They filmed and combed the area, and they knew there was a Red Crescent (facility). The target was the Red Crescent, central and direct."
Russia's Defence Ministry confirmed that a drone had followed the convoy from a warehouse in the government side of Aleppo to its destination in Uram al-Kubra.
The International Committee of the Red Cross said 20 civilians were killed in the attack, many of them killed as they were unloading the trucks. Syrian activists and paramedics had said earlier that the air strikes killed 12.
Witnesses said some of the remains were charred beyond recognition.
Among those killed was Omar Barakat, 38, the local director for the Red Crescent and a father of nine. His brother, Ali Barakat, who was also present at the attack, said it took him three hours to reach Omar, who was trapped in his vehicle.
"I stayed with my face on the floor for about an hour because of the intensity of the strikes," said Mr Barakat. Omar died in an ambulance on the way to hospital.
UN secretary general Ban Ki-moon called it a "sickening, savage and apparently deliberate attack" in his address to world leaders at the UN General Assembly on Tuesday.
"Just when we think it cannot get any worse, the bar of depravity sinks lower," he said, describing the bombers as "cowards" and those delivering aid as "heroes".
The UN's humanitarian agency OCHA announced earlier in the day it had suspended relief convoys in Syria pending a review of the security situation. OCHA spokesman Jens Laerke called it "a very, very dark day for humanitarians across the world".