BEIRUT, Lebanon — A humanitarian aid convoy was attacked in Syria on Monday after the Syrian military declared that a seven-day partial cease-fire was over and immediately began intensive bombardments in rebel-held areas of Aleppo, the divided city that has come to symbolize the ravages of the war.

The convoy attack, military declaration and bombings were the strongest signs yet of the gradual unraveling of a broader agreement between Russia and the United States aimed at restarting peace talks to end the conflict in Syria, which has killed an estimated 500,000 people and displaced millions.

Minutes after the Syrian military declared the cease-fire over around sunset, aerial attacks began pummeling insurgent-occupied neighborhoods of Aleppo, residents reported. The few remaining hospitals were back to overflowing, and rescuers struggled to find people in the dark, with the electricity out. By midnight, 34 people were reported killed.

United Nations officials were dumbfounded by the attack on the convoy of 31 trucks, which was escorted by the Syrian Arab Red Crescent and was carrying food, medicine and supplies bound for rebel-held areas of western Aleppo Province. The convoy was among the first to try to deliver humanitarian aid to these areas, a relief plan permitted under the cease-fire agreement.