BEIRUT, Lebanon — The removal of residents from besieged communities in Syria bogged down again on Sunday after rebels opposed to the agreement set fire to buses that were supposed to carry evacuees, while Security Council diplomats signaled that they had reached a compromise deal to send United Nations monitors to oversee the evacuations.

The feuding in Syria and at the United Nations underlined the complexity of the war in Syria, where coalitions of forces on the ground receive support from a range of opposing backers whose agendas often clash.

Turkey and Russia brokered the initial deal to remove civilians and fighters from the last rebel-held districts of eastern Aleppo, but its implementation has been hampered by parties who opposed the deal and wanted to have residents of other besieged communities evacuated as well.

At the United Nations, Russia had threatened to block a Security Council resolution, drafted by France, that called for sending international observers to Aleppo. But after more than three hours of closed-door negotiations on Sunday, diplomats emerged from the Council chambers saying that they had reached a compromise and that they would vote on it Monday morning.