BEIRUT — Bombs rained down on rebel-held eastern Aleppo for a second straight day Wednesday, pounding a district that houses several medical facilities, including the central blood bank, and forcing Syrian staff and patients in the only remaining pediatric hospital to cower in a basement as buildings collapsed around them.

At least 54 people were killed in airstrikes and artillery shelling across northern Syria, part of a long-anticipated offensive against rebel-held areas announced by Russia, a key ally of Syrian President Bashar Assad. The bombardment hit in besieged neighborhoods of Aleppo, as well as the surrounding countryside and the nearby rebel-held province of Idlib.

Russia said its air raids were only targeting Idlib and the central province of Homs to root out militants of the Islamic State group and Syria’s al-Qaida affiliate. But Syrian warplanes were pounding rebel-held districts of Aleppo, home to nearly 275,000 people.

Residents said the aerial campaign intensified Wednesday. Syria’s Civil Defense, known as the White Helmets, recorded at least 150 raids, more than double the number of attacks on Aleppo on Tuesday.

Resident Modar Shekho said warplanes hadn’t left the skies over his neighborhood since 9 a.m. “The helicopters would leave and the jets would arrive,” he said by telephone.