An attack on a convoy carrying humanitarian supplies in the Aleppo area of Syria appeared to be an air strike, but it was not carried out by coalition jets, US Central Command spokesman Colonel John Thomas says.
"It was certainly not the coalition who struck from the air. It does look like an air strike," Col Thomas told reporters during a telephone briefing on Tuesday.
"The only other entities that fly in Syria are Russia and Syria," he said.
The incident could deal a powerful blow to the ceasefire, the latest attempt to halt a war in its sixth year.
The Syrian Red Crescent said the head of one of its local offices and "around 20 civilians" were killed in Monday's strike, which the Syrian Observatory for Human Rights blamed on Russian or Syrian aircraft.
Russia, which is allied with Syria's President Bashar al-Assad's government, denied that either its air force or the Syrian armed forces was responsible. The Syrian army also denied that it was involved in the attack.
The United Nations has suspended all aid convoys to Syria on Tuesday.
Col Thomas said the ceasefire in Syria was in jeopardy because of increased violence.
"This is not the vision that I think was put in place more than seven days ago, to try to get humanitarian assistance flowing into the area and try to decrease the level of violence," he said.
On Saturday, Russia said that US jets had killed more than 60 Syrian soldiers.
The strike triggered a war of words between Washington and Moscow and further strained the cessation of hostilities.
Col Thomas said a brigadier general would be appointed to investigate the incident.
"If this callous attack is found to be a deliberate targeting of humanitarians, it would amount to a war crime," UN aid chief Stephen O'Brien said in a statement. Peter Maurer, president of the International Committee of the Red Cross (ICRC) called it a "flagrant violation of international humanitarian law".
Surveillance aircraft circled in the sky during the hours before a United Nations aid convoy was struck by warplanes, killing relief workers and destroying 18 trucks of humanitarian supplies, several witnesses say.
Rebels and local rescue workers say the surveillance shows the strikes, which have drawn outrage from the United Nations and Western countries, were deliberate, and gives the lie to Russian and Syrian government denials that they were involved.
"We thought at first something was going to happen because there were four or five planes in the air, but they didn't strike at first," said Abu Shahoud, an opposition activist who was present when the attack took place on Monday.
Hussein Badawi, head of the Civil Defence local rescue service in the town of Urem al-Kubra, was 100 metres from the aid depot when the attack took place. His hand was injured by shrapnel, and he described relentless and sustained air attacks that unleashed carnage on workers unloading aid trucks.
"This is an area that wasn't supposed to be bombed, belonging to an international humanitarian organisation... This area is full of civilians, it's residential," Badawi said.
Damascus and Moscow both say their aircraft were not responsible for attacking the convoy.
Russia said only rebel fighters knew the convoy's location.
But the United Nations says all sides were informed of the convoy's whereabouts and the trucks were clearly marked.
The convoy had been tracked from the skies since it left Aleppo in the morning, a rebel who was part of an escort accompanying it said, adding that fighters had shot at the planes, trying to bring them down, but had missed.
The convoy reached Urem al-Kubra in the late morning to unload wheat flour, winter clothes and blankets, food and educational supplies for 78,000 people.
Drones were circling again from around 6:30pm (local time), said Badawi of the Civil Defence. The strikes began sometime after 7:00pm.
There were conflicting accounts of whether it was jets or helicopters that first opened fire, and of how many strikes, using which weapons, took place.