General Joe Dunford , Chairman of the Joint Chiefs of Staff, said two Russian aircraft were in the area over Orum al-Kubra, a town in Aleppo province, where Monday's air strike on the convoy occurred.
He said planes from the Syrian regime were also in the area, so he couldn't definitely say who dropped the bombs.
"There is no doubt in my mind that the Russians are responsible, I just don't know whose aircraft actually dropped the bomb," Dunford told the Senate Armed Services Committee.
"It was an unacceptable atrocity," Dunford said of the deadly strike that killed around 20 civilians late Monday.
Trucks carrying food and medical equipment from the United Nations and other agencies were unloading aid into a warehouse in Orum al-Kubra at the time of the strike.
Speaking at the same hearing, Defense Secretary Ashton Carter also said the Russians were responsible.
"Whether they conducted it or not -- because they've taken responsibility for the conduct of Syrians by associating themselves with the Syrian regime,"Carter said.
Moscow has strenuously denied responsibility in the strike and offered several differing explanations for what might have happened.
On Wednesday, Russia insinuated the United States might have been involved by claiming a Predator drone was in the area at the time of the strike.
The Pentagon strongly rejected the claim.
Dunford also said he supports calls to ground Syrian regime and Russian aircraft.
A ceasefire that went into effect last week in Syria has collapsed, despite last-ditch diplomatic efforts to salvage it.
The Pentagon was deeply skeptical about the possible cooperation, and Dunford said it would not be "a good idea" to share intel with the Russians.