BEIRUT, Lebanon — With Russia’s backing, the United Nations Security Council voted on Monday to send United Nations observers to monitor evacuations from the besieged Syrian city of Aleppo and to report back regularly.

Russia had threatened to veto an earlier resolution proposed by France, which had sought to place the evacuations under United Nations supervision, but a compromise was reached on Sunday that would allow the monitors to observe after consultations with “interested parties.” The resolution passed unanimously.

That could, in principle, give any number of groups on the ground — including Syrian soldiers and the Shiite militias fighting alongside them — the ability to block access. Fighters from the array of rebel groups, including extremists, could also block access to areas they control. Three questions now loom: Will Russia lean on Syria to allow safe and unimpeded access, as the resolution demands? How long will it take for the monitors to start working? And would they be in place before the evacuations are complete?

“Our objective is: immediately,” Samantha Power, the United States ambassador to the United Nations, said when asked by reporters after the vote when the monitors would start. “They need to get in there and be relevant on those green buses,” the ones used to evacuate civilians and rebels.