Twenty-one people tested positive for coronavirus on Friday aboard a ship that was denied entry to San Francisco Bay after a number of passengers and crew developed flu-like symptoms on the vessel, which had been linked to infections from an earlier voyage.

Vice President Mike Pence, recently appointed as the US government's point man on the coronavirus outbreak, said the cruise ship Grand Princess will be brought to an unspecified non-commercial port where all 2,400 passengers and 1,100 crew will undergo another round of tests.

Pence said all crew members would remain quarantined aboard the vessel, whether or not they test positive, but it was unclear what precisely was in store for passengers who show no signs or symptoms of illness.

“Those that need to be quarantined, will be quarantined. Those that require additional medical attention will receive it,” Pence told reporters in a White House briefing.

“But with regard to the 1,100-member crew, we anticipate that they will be quarantined on the ship, will not need to disembark,” he said.

Pence added: “It's very likely that the crew on the Grand Princess was exposed on two different outings, and we know the coronavirus manifested among the previous passengers.”

Passengers expressed shock and dismay that they were not informed of the test results before Pence announced it, and anguish over the uncertainty of what would happen to them next.

“Everybody is doing the best they can with the information they're given. I mean, why did we not know before the vice president announced it on TV?” Kari Kolstoe, 61, a retired teacher and cancer patient from Grand Forks, North Dakota, told Reuters in a phone interview.

She said she was worried about returning home in time for her next round of chemotherapy. “It's very unsettling.”

On a visit to the headquarters of the US Centres for Disease Control and Prevention in Atlanta, before the initial cruise ship test results were released, President Donald Trump said he would prefer the Grand Princess passengers remained on the ship for the duration of a quarantine.

Otherwise, he said, allowing passengers back onto US soil who might become sick later would end up increasing the number of coronavirus cases in the country.

“I don't need to have the numbers double because of one ship that wasn't our fault,” he said.

Princess Cruises, a unit of the world's leading cruise operator, Carnival Corp, said in a statement that the ship's doctor was “in the process of informing the guests and crew of their individual results. All guests and affected crew will remain isolated in their rooms.”

“Guests will continue to be provided complimentary internet and telephone to stay in contact with their families and loved ones, and the ship's company is working to keep all guests comfortable,” it said.

The Grand Princess predicament was reminiscent of the Diamond Princess cruise liner, also owned by Carnival, that was quarantined off Japan in February and was for a time the largest concentration of coronavirus cases outside China.

Experts have criticised Japanese bureaucrats' handling of the onboard quarantine, as ultimately about 700 people were infected and six have died.

TRIP TO MEXICO Diagnostic test kits were flown by an Air National Guard helicopter on Thursday to the Grand Princess, where medical staff took samples from 46 passengers and crew to determine if they have contracted the respiratory virus, Pence said.

The samples were carried back to a state laboratory in the Bay area, where analysis was completed on Friday. Pence said 21 of the tests came back positive, 24 were negative and one was inconclusive. Of the 21, two were passengers and 19 were members of the crew, the cruise line said.

State and local officials acted to halt the cruise liner after learning people aboard had fallen ill and two passengers who travelled on the same vessel last month to Mexico later tested positive for coronavirus. One, an elderly man from Placer County near Sacramento with underlying health conditions, died this week, the first documented coronavirus fatality in California.

The other, from the Bay area, was described by California Governor Gavin Newsom as gravely sick. Health officials say both individuals likely contracted the virus aboard the ship.

A third passenger from the Mexico trip, a Canadian woman from the province of Alberta, has since been reported by health officials there to have tested positive.

A fourth passenger was reported by Minnesota health officials as that state's first known case, a Ramsey County resident recovering at home.

Pence urged elderly people with serious underlying medical conditions - those who health officials say are most at risk for developing serious and life-threatening illness from coronavirus - “to think carefully about travel.”

The Trump administration is contemplating an advisory discouraging US travellers from going on cruises for the time being, according to four US officials familiar with the situation.

US health officials are also seeking to contact some 2,500 passengers who disembarked the Grand Princess in San Francisco on Feb. 21 after the earlier cruise to Mexico.