Authorities in the city of Tonder said "small clumps" of oil were found on beaches on Romo island, prompting the closure and a ban on swimming.

The source of the pollution was not immediately known. The head of the local emergency services, Torben Godballe, told public broadcaster DR that checks were being carried out.

"Then we must determine how to clean the beach as quickly and effectively as possible," he added. "It's extremely rare to have this kind of situation here."

Romo is the southernmost of Denmark's Wadden Sea islands.