The top US military officer has told a Senate committee he believes Russia bombed the humanitarian aid convoy in Syria on Monday, calling it an "unacceptable atrocity".
The statement from US Marine General Joseph Dunford, chairman of the Joint Chiefs of Staff, was the first time a senior US official publicly pointed the finger at Moscow.
Under questioning from the Senate Armed Services Committee, Gen. Dunford revealed for the first time that both Russian and Syrian aircraft were in the area at the time of the strike.
And when pressed by senators for his own conclusion on the matter, he said he believed Russia was the one that launched the air strike.
US officials initially said the aircraft that dropped the bombs were Russian Su-24 fighter jets, but they were not sure if the aircraft were piloted by Russian or Syrian government troops.
The Obama administration has blamed Russia either way, because of Moscow's continued influence over the Syrian government in the war.
When first asked if Russia bombed the aid convoy, Gen. Dunford said it was not certain which aircraft dropped the bombs, killing 20 civilians.
Under further questioning from Senator Lindsey Graham on whether it was Russia, Gen. Dunford answered, "that hasn't been concluded, but my judgement would be that they did." He was later asked again if it was Russia, and he answered yes.
During a wide-ranging hearing, Gen Dunford also said the Obama administration is considering directly arming the Syrian Kurds whom the US has been backing in the fight against Islamic State.
"They are our most effective partner on the ground," Gen. Dunford said of the battle-hardened Kurds, who are senior partners in an opposition group the US calls the Syrian Democratic Forces.
Gen. Dunford said it is "very difficult" to manage the balance between supporting the Kurds and dealing with the Turkish government's adamant opposition to that support. Turkey sees the Kurds as a long-term political threat.
Asked whether arming the Syrian Kurds is a viable military option, despite the Turkish government's opposition, Gen. Dunford said "reinforcing" the Syrian Democratic Forces' military capabilities "will increase the prospects of our success" in enabling the recapture of Raqqa, the de facto IS capital in Syria.
Meanwhile, a 23-lorry convoy has delivered humanitarian aid to a Damascus suburb besieged by government forces, as the UN resumes its aid deliveries to Syria's opposition areas.
Local media activist Wassim al-Ahmad said the trucks arriving in Moadamiyeh carried medical and food supplies from various UN agencies and the International Committee of the Red Cross.
The Syrian Arab Red Crescent, which supervised the delivery, said the convoy covers the needs of 7,000 families, and another convoy to support 5,000 more will soon follow.