An Australian man has died after falling overboard on Royal Caribbean's Symphony of Seas cruise ship, in spite of rescue efforts the company says were launched immediately after the incident.

The incident occurred early Wednesday morning, as the ship was making its way to St Thomas, in the US virgin islands.

"Our hearts go out to the family and loved ones for their loss, and our care team is providing support and assistance during this difficult time," said Owen Torres, a spokesperson for the company, in a statement provided to the media.

Mr Torres said that the ship turned around immediately after the man, who has not been identified, fell overboard.

Josh Mackey, another passenger on the ship, said that he noticed search lights around 3am, or 4am, but that he did not hear any announcements about the man falling overboard.

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Mr Mackey told ABC News that he later noticed rescue boats focusing on a body that was bobbing in the water, and that the body was eventually taken on board.

The cruise line is now cooperating with investigators as they work to determine how the individual fell overboard from the ship, which is the largest passenger ship in the world when measured by gross tonnage.

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"We are working closely with authorities and will continue to assist in their investigation," Mr Torres said.