Ben Rhodes, a deputy U.S. national security adviser, said the White House held Russia responsible for what he called an "enormous humanitarian tragedy" but he did not address whether the attack was carried out by Russian aircraft.
Earlier Russia, which denied its aircraft or those of its Syrian government allies were involved, had said it believed the convoy was not struck from the air at all but had caught fire because of some incident on the ground.
The Syrian Red Crescent said the head of one of its local offices and "around 20 civilians" had been killed, although other death tolls differed. The attack prompted the United Nations to suspend all aid shipments into Syria.
Senior officials from 23 nations emerged from a one-hour meeting on Syria at a New York luxury hotel with little more than an agreement to meet again, on Friday, about how to end a conflict that has killed hundreds of thousands and driven millions from their homes.
They also differed on the chances of renewing the ceasefire.
"The ceasefire is not dead," U.S. Secretary of State John Kerry said after the meeting, which he hosted with Russian Foreign Minister Sergei Lavrov.
"Is there still a chance this ceasefire will be effective? I can't answer that question," French Foreign Minister Jean-Marc Ayrault told reporters. He said that without a ceasefire there would be a "spiral of war, but we have to be honest, the U.S.-Russian negotiation has reached its limits."
The U.N. Security Council is due to hold a high-level meeting on Syria on Wednesday.
The United Nations, Red Cross and United States had all described Monday's incident as an air strike, implicitly pinning the blame on Russian or Syrian aircraft that fly in the area for breaking the ceasefire with an attack on a humanitarian target.
But the U.N. revised a statement to remove the phrase "air strikes" and replace it with references to unspecified "attacks". U.N. humanitarian spokesman Jens Laerke said the original reference to air strikes was probably a drafting error, saying the U.N. was not in a position to determine if they were air strikes but was sure the convoy was "attacked."