Russia was probably responsible for the deadly bombing of a United Nations humanitarian aid convoy in Syria, American officials said Tuesday, further shredding what remained of a severely weakened agreement between the United States and Russia aimed at halting the war.

Aghast at the attack on Monday night, United Nations officials on Tuesday suspended all aid convoys in Syria, describing the bombing as a possible war crime and a cowardly act.

The suspension was announced as the United Nations was convening its annual General Assembly meetings in New York, where the five-year-old Syria war has become the organization’s most anguishing challenge.

“Just when we think it cannot get any worse, the bar of depravity sinks lower,” Secretary General Ban Ki-moon said in his opening remarks to the gathering, his last as leader of the United Nations after 10 years. Mr. Ban called the attack on the convoy “sickening, savage and apparently deliberate.”