More than 100 Britons are trapped on a cruise ship off California's coast amid fears people on board could have coronavirus.
A passenger on an earlier voyage died and at least one other became infected with the flu-like illness.
Tests are being carried out for suspected cases on the Grand Princess, which is carrying 3,533 people, including 142 Britons - 121 passengers and 21 crew - and everyone has been advised to stay in their rooms.
Another ship owned by Princess Cruises, the Diamond Princess, is still quarantined in Japan after at least 705 passengers and crew caught the bug, resulting in the deaths of six people, including a British man.
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Princess said there were no confirmed cases of coronavirus, also known as Covid-19, on the Grand Princess, which was denied entry to its home port in San Francisco.
Less than 100 people were being tested, said company, which is owned by Carnival Corp and based just outside of Los Angeles.
It added: "Public health officials have advised that no guests will be permitted to disembark until all results have been received.
"Out of an abundance of caution, all guests who have been identified for testing have been asked to remain in their staterooms."
The results are expected to be returned on Friday.
Passengers have seen a gradual shutdown on board the ship, starting with the food buffet being closed as gloved staff wiped down surfaces.
The ship's gym, bar, casino and boutiques were closed, and passengers were told to keep to themselves.
Although no cases have been confirmed, holidaymakers are fearing it will be another Diamond Princess situation.
Kathleen Reid, 67, told Reuters: "My first reaction was, 'Oh, c***. We're going to be quarantined, and maybe get sick.
The retiree, from Granbury, Texas, added: "We don't know what's happening, so we're just kind of in limbo, waiting."
California Governor Gavin Newsom insisted that the Grand Princess remain at sea until passengers and crew complaining of flu-like symptoms during a 15-day round-trip cruise to Hawaii could be tested for possible coronavirus infection.
The US Coast Guard airlifted a batch of diagnostic kits to the ship via helicopter, and public health officials said samples collected would be flown back to a San Francisco Bay Area state laboratory for testing.
Tests were ordered after 35 people aboard the ship had fallen ill and two passengers who had travelled on the same vessel for a voyage last month - between San Francisco and Mexico - later tested positive.
One, an elderly man from Placer County near Sacramento with underlying health conditions, died this week, marking the first documented coronavirus fatality in California.
The other, from the Bay area, was described by Newsom as gravely sick.
Health officials say both individuals likely contracted the virus while they were aboard the ocean liner.
Tests will also be given to dozens of holdover passengers from the Mexico trip who stayed on the ship for the voyage to Hawaii, as well as "guests currently under care for respiratory illness," the cruise line said in a statement.
They will remain quarantined on the ship until cleared by medical staff.
Officials were seeking to contact some 2,500 passengers who disembarked in San Francisco on February 21 after the earlier cruise to Mexico.
One of them, a Canadian woman from the province of Alberta, tested positive for the virus this week, health officials there said.
A presumptive case in Nevada involved a man in his 50s who was "linked to the Grand Princess cruise ship outbreak", according to officials in that state.
Passengers on the current cruise, meanwhile, were forced to make do with a rapidly shrinking choice of amusements.
Having already lost access to many of the ship's favourite attractions - the bar, casino, shops, food buffet and gym - guests were also urged to practice "social distancing," making an effort to keep at least 6 feet away from strangers on the ship, the company said.
By midday on Thursday, they were asked to confine themselves to their staterooms until further notice.
It was unclear what would occur should anyone now aboard the ship test positive for the respiratory virus, which has infected more than 95,000 people worldwide, most of them in China, where the outbreak originated.
Reid, who is travelling with her husband, said the ship's captain was keeping passengers informed of developments with announcements every couple of hours, and that fellow guests seemed to be taking the uncertainty mostly in stride.
She added: "People are, I'm sure, a little anxious, but nobody has just gone screaming mad yet."
She said she had seen no obvious signs of anyone being sick.
"Hand-washing is a big deal," she said, but "nobody is walking around sneezing or coughing."
Princess Cruises has cancelled the next scheduled departure of its Grand Princess Hawaii voyage from San Francisco, which had been set for March 7.
Japanese authorities said last week that a British tourist who had been on board the Diamond Princess had died after contracting coronavirus.
Experts have criticised Japan's handling of the quarantine as a small handful of cases developed into a full-blown outbreak with more than 700 infections.
At the time, it was the largest concentration of coronavirus cases outside China.
