US authorities on Sunday were monitoring dozens of cruise ships hit by Covid-19 cases, with several reportedly denied port in the Caribbean.

Over 60 vessels were under observation after "reported cases of Covid-19 have met the threshold for CDC investigation," the US Centers for Disease Control and Prevention said.

The Washington Post reported that several cruise liners were denied port at their scheduled destinations.

Brenda Hammer, who was set to board the Odyssey of the Seas, a Royal Caribbean cruise ship, told AFP: "I'm a little nervous about it. I wasn't sure I still wanted to come."

Earlier this week, 55 people on the ship tested positive for Covid-19 which spread among passengers and crew members despite 95 percent of people on board being vaccinated, the company said.

Hammer, 69, said that she ultimately decided to go ahead with her vacation because "I already paid my money."

The ship did not dock at the Caribbean islands of Curacao and Aruba, the last scheduled stops on its eight-day voyage out of precaution, before it returned to port at Fort Lauderdale, Florida on Sunday.

"We're just being optimistic about everything, even if we can't get off, we've got all our friends with us and hopefully everything inside is on," said Brij Patel, 45, ahead of boarding.

Another ship, the Carnival Freedom, was turned away from the Caribbean island of Bonaire, the Post reported.

In a statement to AFP, Carnival confirmed that "a small number on board were isolated due to a positive COVID test."

"The rapid spread of the Omicron Variant may shape how some destination authorities with limited medical resources may view even a small number of cases, even when they are being managed with our vigorous protocols," the company said, without providing further details.

The Carnival Freedom arrived in Miami Sunday morning, debarked all guests and "will depart on its next voyage as planned," the company said, adding if it was denied entry to a certain port it would work "to find an alternative destination."

It added the CDC was "fully informed and supportive of our protocols and operational plans."