BEIRUT, Lebanon — At least 80 people were killed on Tuesday in Syrian government air and artillery strikes on besieged suburbs of the capital, Damascus, one of the last rebel-held strongholds. It was the bloodiest day so far in a weekslong escalation that prompted United Nations officials to issue an unusual call for an immediate cease-fire.

The toll, compiled by rescue workers and rising into the night, came as at least six more people were killed in another rebel-held area, in the northern province of Idlib. There, in the past week alone, the government’s Russian-backed air war has damaged several hospitals and clinics and killed dozens of people, including many civilians.

United Nations humanitarian officials declared the situation “extreme” even for the nearly seven-year war, and called on Tuesday for an immediate cease-fire for at least a month to allow aid deliveries.