All hospitals in Syria's besieged opposition-held eastern Aleppo are out of service after days of heavy air strikes, its health directorate and the World Health Organization (WHO) said, though a war monitor said some were still functioning.

White House national security adviser Susan Rice said the United States condemned "in the strongest terms" the latest air strikes against hospitals and urged Russia, an ally of Bashar al-Assad, to take steps to halt the violence.

Intense air strikes have battered the eastern part of the city since Tuesday, when the Syrian regime army and its allies resumed operations after a pause lasting weeks. They launched ground attacks against positions on Friday.

The war monitor, the Britain-based Syrian Observatory for Human Rights, said 48 people, including at least five children, had been killed in eastern Aleppo on Saturday by dozens of air strikes and barrel bombs and dozens of artillery rounds.

That brings the number of people killed by the increased bombardment of Aleppo and the surrounding countryside over the past five days to about 180, including 97 in the city's besieged eastern sector, the observatory added.

Warplanes, artillery and helicopters continued bombarding eastern Aleppo on Saturday, hitting many of its densely populated residential districts, the Observatory said. There were intense clashes in the Bustan al-Basha district, it added.

"This destruction of infrastructure essential to life leaves the besieged, resolute people, including all children and elderly men and women, without any health facilities offering life-saving treatment ... leaving them to die," said Aleppo's health directorate in a statement sent to Reuters late on Friday by an opposition official.

Elizabeth Hoff, the WHO representative in Syria, said on Saturday that a U.N.-led group of aid agencies based over the border in Turkey "confirmed today that all hospitals in eastern Aleppo are out of service".

"SICKENING"

The monitoring group said some hospitals were still operating in besieged parts of Aleppo but said many residents were frightened to use them because of the heavy shelling.

Medical sources, residents and opposition forces in eastern Aleppo say hospitals have been damaged by air strikes and helicopter barrel bombs in recent days, including direct hits on the buildings.

"The United States again joins our partners ... in demanding the immediate cessation of these bombardments and calling on Russia to immediately deescalate violence and facilitate humanitarian aid and access for the Syrian people," Rice said in a statement.

British Foreign Secretary Boris Johnson tweeted that reports of air strikes hitting civilians and hospitals in east Aleppo were "sickening" and called for a return to diplomacy.

However, with the United States awaiting the inauguration in late January of President-elect Donald Trump, who has been critical of Washington's Syria policy without laying out detailed plans himself, diplomatic efforts appear stalled.

Both Russia and Assad's regime have denied deliberately targeting hospitals and other civilian infrastructure during the war, which began in 2011 and was joined by Russia's air force in September 2015.

The charity Doctors Without Borders said in a message there had been more than 30 hits on hospitals in eastern Aleppo since early July. "Doctors are few and medical supplies are depleted, with no possibility of sending more supplies in," it said.

Health and rescue workers have previously been able to bring damaged hospitals back into operation but a lack of supplies is making that harder.

The Syrian war pits Assad and his allies Russia, Iran and Shi'ite militias against Sunni opposition forces including groups supported by the United States, Turkey and the Gulf monarchies.

Aleppo, for years split between an opposition-held east and regime-held western sector, has become the fiercest front.

An army offensive backed by a major aerial bombardment from late September to late October killed hundreds, according to the United Nations, and tightened the siege, leaving eastern Aleppo with little food, medicine or fuel.

PHOTO CAPTION

A still image taken from a video posted on social media said to be shot on November 14, 2016, shows the damage inside a hospital, said to be in the opposition-held town of Atareb in the countryside west of Aleppo, Syria.

Reuters