(Updates number of cases, adds quote, background)
OSLO, July 31 (Reuters) - Four crew members on Norwegian cruise vessel Roald Amundsen have been diagnosed with COVID-19, the University Hospital of North Norway where they are being treated said on Friday.
All 160 crew members have been quarantined on the ship, while passengers now off it were being contacted by telephone and told to self-isolate, operator Hurtigruten said.
The vessel had close to 200 passengers on board when it arrived at the Arctic port of Tromsoe early on Friday, all of whom had disembarked, public broadcaster NRK reported.
"By the time we were notified, the passengers had left the ship," Tromsoe municipality chief doctor Kathrine Kristoffersen told a news conference. "All passengers will, for the time being, be asked to quarantine."
Hurtigruten, which in March suspended most operations due to the pandemic, announced on July 7 it would gradually return all but two of its 16 vessels to service by the end of September, albeit with reduced capacity.
The Roald Amundsen had been on a week-long journey from Tromsoe to the Svalbard archipelago, half-way between Europe and the North Pole, and is scheduled to sail around the British Isles in September, docking at ports in England and Scotland.
In the early phase of the pandemic, thousands of passengers were stranded on cruise vessels in waters of Asia, the United States and elsewhere, leading operators to cancel journeys and triggering mass layoffs by the industry. (Reporting by Terje Solsvik; Editing by Andrew Cawthorne)