Washington has said the treatment of Glavprodukt, the only U.S. company Moscow has seized, will influence a planned reset of U.S.-Russia relations.

Russia and North Korea are meeting for a three-day visit that is already sending other countries into a tailspin, as World War 3 fears mount.

The Wonsan Kalma resort was opened in a grand ceremony last month by North Korea's leader, Kim Jong Un, who hailed it as a "world-class tourist and cultural destination". The details of how this resort was built have been shrouded in secrecy in a country largely closed to the outside world.

South Korea sent six North Koreans back across their sea border, months after they drifted south in wooden boats in March and May.