China has imposed new restrictions on imports of Norwegian salmon citing fears of a fish virus, Norwegian authorities said on Monday.

As of Monday, Beijing has banned imports of whole salmon from three Norwegian regions amid concerns they carry infectious salmon anaemia (ISA). That ban represents one-fifth of Norway's salmon exports to China.

For salmon from other regions of the Scandinavian country, Chinese authorities have imposed stricter controls aimed at detecting ISA and pancreas disease.

Norway's food safety agency was informed by letter of the decision, which it judged to be baseless but which strikes yet another blow to the country's star export product.

Sales of Norwegian salmon to China plummeted after the 2010 Nobel Peace Prize was awarded to Chinese dissident Liu Xiaobo.

Norway's food safety agency stressed meanwhile that Norwegian salmon was safe.

"We believe there is no risk that Chinese salmon will be contaminated by the ISA virus because fish products from Norway go directly to consumption," it said in a statement last week.

"The ISA virus is not harmful to humans," it added.

The awarding of the Nobel Prize to Liu plunged the two countries' bilateral ties into a deep-freeze, and led to a tightening of veterinary controls on Norwegian salmon.

Its share of the Chinese market dropped from 90 to less than 30 percent.

The new restrictions come on the heels of an import ban announced by Russia -- another important market for Norway -- in response to western sanctions against Moscow over the Ukraine crisis.