The US Centers for Disease Control and Prevention has warned Americans to avoid cruises regardless of vaccination status due to concern over Covid-19.

On Thursday, the CDC said the virus can quickly spread in the confined spaces of a ship and that the likelihood of infection is high.

It has elevated its travel warning for cruise ships from level three to four - the agency's highest.

The CDC is currently monitoring over 90 cruise ships for Covid-19.

The agency said there has been an increase in cases on cruise ships since identification of the Omicron variant. It warned that even fully vaccinated passengers who have received a booster dose can spread the virus while onboard.

The CDC also recommended that cruise ship passengers be tested between one and three days prior to their voyage, and three to five days after, regardless of vaccination status.

It added that unvaccinated passengers should self-isolate for a week after disembarking.

On Wednesday, the CDC said it was monitoring 92 cruise ships for Covid-19 amid a spike in cases across the US.

It maintains a colour-coded chart for cruise ships on its website based on surveillance data from the previous seven days.

Ships that are marked "red status" are believed to contain a "sustained transmission" of the virus on-board.

As of Thursday, no vessels were marked with the red status. The vast majority were classified as yellow, meaning that at least one case of Covid-19 among crew has been detected.

Yellow status also means that a particular vessel meets the threshold for further CDC investigation.

Reuters reported on Thursday that one vessel - the Queen Mary 2 - will remain in Barbados and not return to New York as a precautionary measure as it brings on additional staff.

The cruise industry was one of the sectors worst hit by Covid-related shutdowns in 2020.

The industry's largest company, US giant Carnival, reported losses of $10bn (£7.4bn) in 2020 after revenues fell 73% during the pandemic.