Image copyright AFP Image caption The Costa Atlantica had no passengers on board and diverted to Japan for repairs

Japan is investigating a virus outbreak among the crew of a cruise ship which docked in Nagasaki in January with no reported cases until a spike this week.

The Costa Atlantica now has 91 crew out of 623 who have tested positive so far, with one in a critical condition.

The ship had no passengers on board and diverted to Nagasaki for repairs rather than China due to the virus outbreak.

The crew were meant to have been confined to the ship but local media report some left the vessel.

Local authorities were first alerted to the possibility of the virus on board the Costa Atlantica last weekend. All crew are being tested - their nationalities are not known.

Media playback is unsupported on your device Media caption Rupert Wingfield-Hayes sees how one Japanese hospital is dealing with coronavirus

"We think we will have a clearer picture once we get all the samples," Katsumi Nakata, head of the regional government's health and welfare department, is quoted as telling the AFP news agency.

"Considering the limited medical resources we have in this region, it is difficult for us to maintain and control the health of all the 630 people.

"We need to maintain the medical system for local residents," he said.

The Nagasaki Medical Association declared a "medically critical situation" on Thursday.

Nagasaki, a port city in southern Japan, has not seen major coronavirus outbreaks and, as the crew were meant to have been confined to the ship, how such an outbreak could have developed is now the subject of investigation.

The vessel arrived in Nagasaki on 29 January and was quarantined but no infection was found at the time. The authorities told crew they could not go beyond the quay unless it was to hospital, Reuters reports.

But NHK says crew members ventured into town even after local authorities asked them to stay on board. A coach company and a taxi firm confirmed they had ferried crew around in late March, and officials suspect crew left the ship on other occasions too.

It is also possible that fresh crew who were taken onboard in Nagasaki brought the virus with them.

NHK reported that about half of the crew have been tested so far. Doctors sent by the Health Ministry and Japan's armed forces are assisting in the task.

Japan came in for criticism for its handling of another coronavirus crisis on a cruise ship - the Diamond Princess - back in February.

After a former passenger tested positive, it was kept in dock in Yokohama and its passengers and crew quarantined on board. More than 700 tested positive and 13 died, according to Johns Hopkins University data.