SINGAPORE:Nearly 1,700 passengers on a Royal Caribbean 'cruise-to-nowhere' from Singapore were told to stay in their cabins on Wednesday after a COVID-19 case was detected on board, forcing the Quantum of the Seas ship back to port, authorities said.

All passengers had cleared a mandatory polymerase chain reaction test for COVID-19 up to three days before the cruise started on Monday, the Singapore Tourism Board (STB) said.

The infected passenger, an 83-year-old male, had reported to the onboard medical centre with diarrhoea, and others on board were told of the infection early on Wednesday.

Royal Caribbean and the STB said all guests and crew of the ship who had close contact with the infected guest had subsequently tested negative for the virus.

The passengers and crew will stay onboard in their rooms until contact tracing is complete, Annie Chang, director of the cruise segment at the STB said. They will all undergo mandatory COVID-19 testing before leaving the terminal.

In the meantime, they are being given regular updates and meals are provided directly to their rooms.

Passenger Melvin Chew told Reuters he was awoken by an announcement on the ship's tannoy just before 3 a.m. Singapore time on Wednesday (1900 GMT Tuesday) which said a guest had tested positive and all passengers must remain in their rooms.

"I was like: 'there it goes, the worst fear has happened'," said Chew, a 31-year-old business development manager, who had taken the cruise with a friend.

The Quantum of the Seas returned to Singapore at 8 a.m. local time (0000 GMT), and as of 2 p.m. passengers were still being asked to isolate in their cabins.

The 'cruise-to-nowhere' by Royal Caribbean is one of its first sailings since the company halted global operations in March due to the coronavirus. There were 1,680 guests and 1,148 crew members on board, a spokesperson for Royal Caribbean said.

The global cruise industry has taken a major hit from the pandemic, with some of the earliest big outbreaks found on cruise ships. In one case in February off the coast of Japan, passengers were stuck for weeks aboard the Diamond Princess with over 700 guests and crew infected.

Royal Caribbean's 'cruise-to-nowhere' is open only to Singapore residents, makes no stops and sails just off the city-state.

The cruises are a part of Singapore's plans to revive its tourism industry that has been battered due to the novel coronavirus, which has infected more than 67.7 million people globally and killed 1,548,575​.

Singapore, which has had just over 58,000 cases and 29 deaths, has been reporting less than a handful of local infections in recent weeks.

The case on board is another setback for Singapore after a plan to open a quarantine-free air travel bubble with Hong Kong last month was postponed at the eleventh hour.

Part of the precautions for the resumption of cruises in Singapore involved the pre-departure testing and for guests to carry an electronic contact tracing device and to social distance at all times.

"I really don't know how the patient has gotten it," said another passenger Muhammad Rezal Ramli, 40, who had taken the cruise with his two young children.

"Even if we get the disease, it's not something that we can complain about because we are aware of the risk...when we signed up for the cruise... There's always this possibility."

It was not immediately clear when passengers would be allowed to disembark.

The infected case's close contacts will be placed in quarantine or health surveillance, according to an advisory from the health ministry sent to passengers.

Others will need to monitor their health, while continuing regular activities including going to school or work, and undergo a swab test at the end of a 14-day monitoring period.