* US demands immediate end of bombardments
* Observatory says at least 27 killed on Saturday
* Assad's military, Russia deny targeting hospitals (Adds US condemnation, death toll from Saturday bombing and Doctors Without Borders comment)
By Angus McDowall
BEIRUT, Nov 19 (Reuters) - All hospitals in Syria's besieged rebel-held eastern Aleppo are out of service after days of heavy air strikes, its health directorate and the World Health Organisation (WHO) said, though a war monitor said some were still working.
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 Syrian President 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 army and its allies resumed operations after a pause lasting weeks. They launched ground attacks against insurgent positions on Friday.
The war monitor, the Britain-based Syrian Observatory for Human Rights, said at least 27 people, including children, had been killed in eastern Aleppo on Saturday by dozens of air strikes and barrel bombs and dozens of artillery rounds.
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".
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 rebels 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.
Both Russia and Assad's government 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 rebels including groups supported by the United States, Turkey and the Gulf monarchies and also jihadist groups.
Aleppo, for years split between a rebel-held east and government-held western sector, has become the fiercest front. During the summer, pro-government forces managed to besiege the districts held by insurgents which are home to about 270,000 people, according to the United Nations.
Syrian state television said on Tuesday the air force had targeted "terrorist strongholds and supply depots" in Aleppo. Russia has said its air force is only conducting air strikes in other parts of Syria. The Damascus government describes all the rebels fighting it as terrorists.
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.
A rebel counter-attack early this month involved shelling that killed dozens of civilians, the U.N. said, but it quickly petered out and the army and its allies, including Hezbollah and Iraqi militias, reversed all insurgent gains in about two weeks. (Editing by Gareth Jones)
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