BEIRUT (AP) — A wave of nighttime airstrikes hit a hospital in Syria supported by Doctors Without Borders in the rebel-held part of the contested city of Aleppo, killing at least 27 people.

The U.N. envoy for Syria appealed early Thursday on the U.S. and Russia to help revive the peace talks and a cease-fire, which he said "hangs by a thread."

The Civil Defense, a volunteer first-responders agency whose members went to the scene of the attack, put the death toll at 30 and said the dead included six hospital staff. Among those slain was one of the last pediatricians remaining in opposition-held areas of the contested city and a dentist. The strikes, shortly before midnight Wednesday, hit the well-known al-Quds field hospital in the rebel-held district of Sukkari in Aleppo, according to opposition activists and rescue workers.