Athens - Five people discovered with swine flu aboard a cruise ship that docked in Greece's main harbour Friday have been put in quarantine, officials said.

Health Ministry officials said the four crew members and a passenger were among 4,500 people aboard the Ruby Princess, operated by Princess Cruises.

"We have advised the four crew and passenger to remain in their cabins and to not disembark," said Health Ministry official Panagiotis Efstathiou, adding that all the other passengers have been allowed to leave the vessel.

The captain had asked the Greek government to inspect the ship as soon as it arrived in the port of Piraeus from neighboring Turkey.

The Ruby Princess was scheduled to leave for Venice, Italy on Friday evening.

Since May 18, when Greece confirmed its first case of swine flu, more than 520 people have undergone treatment.

A 33-year-old male hospitalised in Heraklion of the southern Mediterranean island of Crete is in serious condition. (dpa)