A U.S. Navy sailor was medically evacuated Saturday afternoon from an American nuclear-powered submarine by Danish military forces, according to a U.S. and Danish official.

The submarine broke from its mission and surfaced about eight miles from Nuuk, Greenland, an extraordinary step for a vessel designed to remain hidden beneath the sea in secrecy.

The sailor was airlifted by a Danish Defense Seahawk helicopter, deployed from the Vædderen, a Danish military patrol ship.

It's unclear what the nature of the medical emergency was, but it was not combat-related, according to the U.S. official.

The sailor is being treated at a hospital in Nuuk, Greenland's capital.