The convoy of 31 trucks was being escorted by the Syrian Arab Red Crescent and was carrying supplies bound for rebel-held areas of western Aleppo Province when it was hit.

The United States said it was “outraged” by the attack. John F. Kirby, spokesman for the State Department, said the convoy’s movements had been known to the Syrian authorities and to Russian officials. He added that the United States would reassess the prospects for cooperation with Russia in light of this “egregious violation” of the week-old cessation of hostilities.

The airstrike came as workers were unloading aid. It killed a senior official of the Syrian Arab Red Crescent and some civilians, but initial reports that 14 people had died could not be confirmed, said Benoit Carpentier, a spokesman for the International Federation of Red Cross and Red Crescent Societies.

Repeated strikes by aircraft destroyed 18 of 31 trucks that the United Nations said had been clearly marked as a humanitarian convoy. The trucks were carrying wheat flour, nine tons of medicine and winter clothing for about 78,000 people. The bombs also destroyed a hospital, Mr. Carpentier said. The attack came shortly after the Syrian Army had announced that the partial cease-fire was over and resumed offensive operations, reportedly including airstrikes on rebel-held parts of the city of Aleppo.

Aid convoys have run the gantlet of sniper fire and shelling in the course of the five-year conflict, but the attack on Monday is believed to have been the first time a convoy has come under attack by aircraft. Aid agencies did not say whether the planes were Syrian or Russian.