BEIRUT — The United Nations faces “a problem” in shipping humanitarian aid into Syria, the U.N. envoy for the war-torn country said Thursday, pinning the blame on the lack of authorization from Bashar Assad’s government that has even disappointed Russia, the Syrian president’s key backer.

Staffan de Mistura said a U.S.-Russia-brokered cease-fire deal agreed on last week has largely reduced the violence since it came into effect on Monday, but the humanitarian aid flow that was expected to follow has not materialized.

De Mistura said 40 aid trucks are ready to move and that the U.N. would prioritize delivery to the embattled, rebel-held eastern neighborhoods of the northern city of Aleppo.

However, the Syrian government has not provided needed “facilitation letters,” or permits, to allow for the start of the convoys, de Mistura said. He said the government had agreed on Sept. 6 — before the cease-fire deal was inked — to allow aid into five areas, but the authorizations still haven’t come.

Aside from the reducing the bloodshed, the “second dividend” of the U.S.-Russia deal is humanitarian access, de Mistura told reporters in Geneva. “That is what makes a difference for the people, apart from seeing no more bombs or mortar shelling taking place.”

“On that one, we have a problem,” he added. “It is particularly regrettable … These are days which we should have used for convoys to move with the permits to go because there is no fighting.”

“The Russian Federation is agreeing with us,” he said.

Jan Egeland, the top humanitarian aid official in de Mistura’s office, said the “good news” from the cessation of hostilities was that the bloodshed has dropped — and that “attacks on schools, attacks on hospitals have stopped.”