Denmark's prime minister announced special restrictions for more than 280,000 people in the northwest of the country on Thursday after a mutated version of the new coronavirus linked to mink farms was found in humans.

Copenhagen warned that the mutation could threaten the effectiveness of any future vaccine.

"From tonight, citizens in seven areas of north Jutland are strongly encouraged to stay in their area to prevent the spread of infection," Mette Frederiksen told a news conference, adding that people were being ordered not to travel there, while bars and restaurants would also shut.

"We are asking you in north Jutland to do something completely extraordinary," Frederiksen said, talking of a "real closure" of the region.

"The eyes of the world are on us," she added.

Public transport in the region will be shut down with buses and trains stopped from entering or leaving.

Denmark, the world's largest exporter of mink fur, raised concerns on Wednesday by announcing the slaughter of all mink in the country -- numbering 15 to 17 million spread over 1,080 farms -- following the discovery of the mutation which can be passed to humans.

The mutation has already been detected in 12 people -- 11 cases in the region being closed down, and one in another.