Beirut — As Syrian government airstrikes pounded eastern Aleppo on Wednesday, people huddled around incubators in the basement of the city’s last remaining children’s hospital, waiting in silence as they counted the bombs.

By midday, they said, the tally had passed 20, shaking the ground and shattering equipment in the rooms above. Speaking from their underground refuge, the hospital’s director said he and his staffers were trapped.

The attacks continued as night fell, marking the second day of a major offensive to retake Aleppo’s eastern districts from armed opposition groups. Defeat in Aleppo, once Syria’s economic powerhouse, would be a major blow to the rebels. It could also hasten the fall of their remaining strongholds across the country.

The offensive came a day after government forces and their Russian allies resumed attacks across northern Syria, including Russian cruise missile strikes from a warship in the Mediterranean.

Monitoring groups said at least 60 people have been killed in two days in Aleppo province. The number is expected to rise significantly as attacks intensify and underequipped medical facilities struggle to cope.

“The planes are up above. We can’t get out,” said the director of the children’s hospital, who gave his name only as Hatem. “We’ve been hit before, but this isn’t like the other times. Every mother is holding her child. We can’t control things anymore. The parents want to protect their children by themselves.”

Residents said eastern Aleppo’s central blood bank, located close by, also was hit.

Video footage showed the aftermath of the fierce bombardment. Fires licked the edges of cars and buildings, and the air was filled with dust.

The Independent Doctors Association, a nonprofit group that supports the children’s hospital, said it was badly damaged.

When they finally left the basement under cover of darkness, staff at the children’s hospital found that the bombing had given way to shelling. And as nurses ushered patients back to their beds, the doctors prepared for a long night ahead.

“We’re safe, but we have to tidy up now. This will only happen again,” said Hatem, the director.

It was the sixth such attack on a medical facility in opposition-held areas nationwide in three days. The United States on Tuesday condemned the previous attacks, calling them violations of international humanitarian law.

The bombardment ended a period of relative respite, particularly in eastern Aleppo, where Moscow halted airstrikes on Oct. 18 ahead of brief cease-fires. Although residents were ordered to leave for their own safety, few did so. Many expressed fears about what awaited them outside.

Russia said on Tuesday that it had launched a major offensive against rebel-held areas in Idlib and Homs provinces, but it denied involvement in the new attacks on Aleppo.

In Moscow, a spokesman for the Defense Ministry, Maj. Gen. Igor Konashenkov, said Russian warplanes have “carried out no strikes against the city of Aleppo for 29 days now,” according to the Interfax news agency.