GENEVA — The government of President Bashar Assad is holding up deliveries of aid in violation of the deal struck last week by Russia and the United States, the UN mediator for Syria said Thursday.

The mediator, Staffan de Mistura, told journalists the cessation of hostilities agreement that took effect Monday after several weeks of negotiations was largely holding.

He said, however, that the delivery of humanitarian aid that was part of the agreement reached by Russia and the United States had not taken place because Assad’s government had not issued the authorization letters that aid convoys use to pass through checkpoints.

“It’s particularly regrettable because we are losing time,’’ said de Mistura, emphasizing the importance of getting the convoys moving during the weeklong cease-fire and calling for immediate action to unblock aid deliveries. “The Russians were as disappointed as we are’’ and called for quick action on the matter, he said at the end of a meeting of the humanitarian task force monitoring developments.

A UN convoy of 20 trucks has crossed the Turkish border into a buffer zone with food and other supplies destined for the rebel-held eastern part of Aleppo, but it has also been held up, though for a different reason: An agreement to separate the forces along the road into the city had yet to take effect, de Mistura said.

Under the terms of the Russian-US agreement, the convoy needs only to notify the government; it does not need formal permission from the Syrian government.

The Syrian Observatory for Human Rights, a monitoring group based in Britain with a network of Syrian contacts, and Al Mayadeen, a Lebanese news channel that leans in favor of the Syrian government, both reported that the convoy had entered opposition-held territory in Aleppo province, and the observatory said it had entered the city itself. There was no immediate confirmation from people in eastern Aleppo.

The United Nations was also ready to send aid by the end of the week to besieged areas close to Damascus and Homs, including Moademiya, al-Waer, Talbish, and Douma, according to Jan Egeland, the UN special adviser on humanitarian affairs.

The government had approved aid deliveries to those locations before the cessation of hostilities but had failed to issue a single letter of authorization, Egeland said.

“Can well-fed, grown men please stop putting political, bureaucratic, and procedural roadblocks for brave humanitarian workers that are willing and able to go to serve women, children, wounded civilians in besieged areas?’’ he asked.

The Observatory also reported strikes in Islamic State-held territory in the eastern province of Deir el-Zour had killed at least seven people and wounded 30 others. The strikes hit four buildings, the group said, one of them a school. It was unclear whether the strikes came from the United States, Russia, or the Syrian government.