UNITED NATIONS — Five days after world powers announced that aid would soon reach starving Syrians trapped behind front lines, the United Nations was still negotiating with the government in Damascus on Tuesday over lifting blockades to humanitarian convoys, announcing by day’s end that it hoped to start sending 80 trucks to deliver food and lifesaving medicines on Wednesday.

The United Nations special envoy for Syria, Staffan de Mistura, said in an email that the government had agreed to lift its sieges on five towns, so as to allow aid convoys that have not been permitted to enter for months.

“It is clear it is the duty of the government of Syria to want to reach every Syrian person wherever they are and allow the U.N. to bring humanitarian aid,” Mr. de Mistura said earlier in a statement sent from Damascus, after his meeting with the Syrian foreign minister, Walid al-Moallem. “Tomorrow we test this.”