BEIRUT — Government bombardment of besieged rebel-held neighborhoods in Aleppo killed at least 20 people Saturday, the worst since airstrikes resumed earlier this week, said Syrian opposition activists, as the U.S. called for an end to the bombings.

Saturday was the fifth day of renewed assaults by Syrian warplanes on eastern Aleppo districts, a rebel-held enclave of 275,000 people. The onslaught began Tuesday, when Syria’s ally Russia announced its own offensive in central Syria.

The bombing on Saturday came after a day of airstrikes that hit four hospitals in east Aleppo.

A statement issued late Friday by the opposition’s Aleppo Health Directorate said that all hospitals in east Aleppo are out of service because of the bombing over the past days.

“The intentional destruction of infrastructure for survival has made the besieged steadfast people, including children, elderly and men and women, without medical facilities to treat them,” the statement said.

The Britain-based Syrian Observatory for Human Rights said not all hospitals in east Aleppo neighborhoods are out of service but people are finding difficulties reaching them because of the intensity of the shelling.

The White House meanwhile demanded an immediate halt to Syrian strikes on eastern Aleppo. Speaking in Peru, White House National Security Adviser Susan Rice said the U.S. condemns the “horrific attacks” against hospitals and aid workers “in the strongest possible terms.”

The White House is putting the onus on Russia to lower the violence and help humanitarian aid get to besieged Syrians.

Opposition activists said Saturday’s death toll has been the worst since the aerial campaign resumed on Tuesday. Residents said hundreds of artillery shells and dozens of airstrikes have hit the city, increasing the misery of its residents who have been suffering from lack of food and medicine because of the siege imposed by government forces and their allies in July.

“Aleppo is being wiped out in front of the eyes of the world,” medical official Mohammed Abu Rajab said. “It’s not only hospitals that are out of service. All liberated areas in Aleppo are out of service.”

The Observatory said Syrian planes and artillery struck more than 20 neighborhoods in east Aleppo killing 27 people and wounding many others.