A Russian navy intelligence ship has been in the Pacific off Japan's southern Okinawa island, possibly to monitor radio wave data from US forces, Japan's Kyodo News reported Friday.

A Japanese destroyer found the ship moving and stopping repeatedly in the high seas some 30-60 kilometers (18-37 miles) east of the island of Okinawa on Wednesday, the report said citing Japanese Defense Ministry sources.

The Russian vessel stopped its engines and began floating on Thursday afternoon at a point some 70 kilometers east of the southern tip of the island, Kyodo said.

More than half of the 40,000 US troops in Japan are based in Okinawa.

US troops are stationed in Japan under a security treaty with the country, which has been constitutionally pacifist since World War II.