The Trump administration has declared a public health emergency over the coronavirus outbreak, and said it would bar entry to the United States starting on Sunday of foreign nationals who have traveled to China.

US citizens who have traveled to China's Hubei Province within the last 14 days will be subject to a mandatory 14-day quarantine, Health and Human Services Secretary Alex Azar told a media briefing at the White House.

The administration will also limit flights from China to seven US airports, he said.

Earlier, UK health officials said two people have tested positive for the coronavirus in England, in what are the first cases to reach Britain since the deadly outbreak emerged in China and spread globally.

The announcement came as 83 British citizens arrived back on a UK government-chartered flight from Wuhan, the Chinese city at the centre of the epidemic which has so far killed 213 people.

The plane carrying them, as well as 27 other foreign nationals, landed at the Brize Norton military airbase in south central England shortly after 1.30pm.

EU citizens on board will continue on to Spain while British families disembarking were set to be isolated for 14 days at a medical facility in the country's northwest.

With a growing number of cases reported in at least 19 other countries, the World Health Organisation on Thursday declared an international public health emergency.

Chris Whitty, Chief Medical Officer for England, insisted the country was "extremely well-prepared" and that officials were working with the WHO and others "to ensure we are ready for all eventualities".

"We can confirm that two patients in England, who are members of the same family, have tested positive for coronavirus," he said in a statement.

Prof Whitty added the patients were receiving specialist care, including "tried and tested infection control procedures to prevent further spread of the virus".

He added officials were working to identify any contacts the two patients had.

AFP understands the pair had travelled to China recently. UK health officials declined to reveal the location, citing patient confidentiality.

It is understood they are being treated at a hospital in Newcastle, northeast England.

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"It is not surprising that we now have two confirmed cases in the UK as we live in a global world," said Devi Sridhar, Professor of Global Public Health at the University of Edinburgh.

"This is still primarily a Chinese emergency as 99% of cases have been in China and all deaths have been in China," he added.

China's National Health Commission revealed this morning that nearly 10,000 people there have been infected by the virus.

Wuhan, a city of 11 million people, has been subject to an unprecedented lockdown, preventing residents from leaving in a bid to stop further spread.

Thousands of foreigners were among the millions of people confined in the metropolis, and numerous countries have begun airlifting their nationals out.

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Japan has urged its citizens to avoid non-essential travel to China, following similar warnings by Germany, Britain and other nations in recent days.

Those warnings are not as definitive as the US directive against all travel to China.

Among the array of other extraordinary containment efforts, many major airlines this week suspended or reduced flights to China.

Mongolia also halted cross-border traffic with its huge neighbour and Russia sealed its remote far-eastern frontier.

Italy and Israel barred all flight connections with China. Impoverished Papua New Guinea went so far as to bar all visitors from "Asian ports".

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The US this week reported its first case of person-to-person transmission of the virus on American soil - a man in Chicago who got it from his wife, who had travelled to Wuhan.

In a sign of growing global fears, more than 6,000 tourists were temporarily confined aboard their cruise ship at an Italian port after two Chinese passengers fell ill. They later tested negative for the coronavirus.

A pilot union in the United States sued American Airlines to demand it halt all flights to China.

British diplomatic staff had been "working round the clock" to organise today's airlift, and had expected to evacuate up to 200 people but only 110 boarded.

"We will work with our international partners on further assistance for those who remain," a spokesman for Prime Minister Boris Johnson said.

The British families arriving at the Royal Air Force's Brize Norton base will be ferried by bus for the two-week quarantine at Arrowe Park Hospital in The Wirral in northwest England.

They will stay in accommodation usually used by health service staff, and will have access to the internet.

Anyone with suspicious symptoms will be taken to the nearby Royal Liverpool and Broadgreen Hospital, which has a high-level infectious diseases unit.

"It's welcome news that our evacuation flight has now left Wuhan," Foreign Secretary Dominic Raab said in a statement.

"We know how distressing the situation has been for those waiting to leave."