NEW YORK (NYTIMES) - The US State Department on Sunday (March 8) advised Americans against travelling on cruise ships, warning that they presented a higher risk of coronavirus infection and made United States citizens vulnerable to possible international travel restrictions, including quarantines.

The decision came after President Donald Trump resisted requests from administration officials to publicly urge older travellers to avoid cruise ships and plane travel, saying he thought it would harm those industries, according to two people familiar with the discussions.

"US citizens, particularly travellers with underlying health conditions, should not travel by cruise ship," the State Department wrote in an alert posted to its website on Sunday.

Americans should not rely on being evacuated if other countries subject them to quarantine, the department said.

The guidance signalled another escalation in the Trump administration's efforts to ward off the fast-growing spread of the virus, and another instance of what appeared to be separation between health officials and the president, who has repeatedly registered scepticism over making statements he believes are alarmist.

It came as federal and state officials scrambled to contain a cluster of cases on the 3,500-passenger Grand Princess, which has idled off the coast of California. Twenty-one people on board tested positive for the coronavirus, and it was on its way to dock on Monday at the Port of Oakland, the vessel's operator said.

The decision to caution Americans capped weeks of discussions on the Trump administration's inter-agency task force about how to communicate alarm about cruise ships after hundreds of Americans, including some who tested positive for the virus, were evacuated from the Diamond Princess off the coast of Japan, a process rife with confusion and disorder.

It was Vice-President Mike Pence, who is overseeing the federal government's coronavirus efforts, who signed off on the State Department's announcement, and Mr Stephen Biegun, the deputy secretary of state and another task force member, who formalised it, a senior official familiar with the decision said. The final call on Sunday did not reach Mr Trump's level, the official said.

The White House did not immediately respond to an e-mail seeking comment.

According to an official with knowledge of the talks, at least four members of the task force pushed for the State Department to caution Americans against cruises: Dr Robert Redfield, the director of the Centres for Disease Control and Prevention; Dr Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases; Dr Robert Kadlec, the assistant health secretary for preparedness and response; and Dr Deborah Birx, the new coronavirus response coordinator.

The talks picked up last week as the Grand Princess waited for a confirmation of possible infections on the ship. A 71-year-old man died after travelling on a previous leg of the cruise, a round trip from San Francisco to Mexico. Another passenger from that leg of the trip also tested positive for the virus and is being treated in California.

After the Grand Princess docks, those aboard will be taken to military facilities around the country to be tested and quarantined for 14 days, according to the Department of Health and Human Services.

About 1,000 passengers who are California residents will go to Travis Air Force Base in Fairfield, California, or the Marine Corps Air Station Miramar in San Diego. Residents of other states will be taken to Joint Base San Antonio-Lackland in Texas or Dobbins Air Reserve Base in Marietta, Georgia.

More than 500 coronavirus infections have been confirmed by lab tests in the United States as of Sunday, and 22 people have died so far from them.

"This is a fluid situation," the State Department notice said, adding that the CDC recommends that older adults, in addition to those with underlying health problems, should avoid any setting that makes them more vulnerable to disease.

"This entails avoiding crowded places, avoiding non-essential travel such as long plane trips, and especially avoiding embarking on cruise ships," the alert said.

In response to the State Department, Carnival Corp said in a statement on Sunday that its brands, which include Carnival Cruise Line, Holland America Line, Princess Cruises and Cunard, enhanced its health screening protocols to include thermal scans and temperature checks before boarding and on board.

"We are currently in discussions with the CDC, World Health Organisation and other health officials," the statement said, noting that the cruise industry committed to an aggressive response plan when it met Mr Pence on Saturday.

"The health and safety of our guests and crew is our highest priority, and cruising remains one of the most attractive vacation options available."

On Sunday morning, Dr Fauci appeared to preview the State Department's announcement, saying on Fox News Sunday that older Americans in poor health need to avoid large crowds and plane travel. "Absolutely don't get on a cruise ship," he said.