An information-gathering ship of the Russian navy on Wednesday briefly entered a strip of sea just outside territorial waters off the Okinotori Island, Japan's southernmost island in the Pacific, the Defense Ministry said.
It marked the first time for the ministry to make public information related to the movements of a Russian naval ship within the so-called contiguous waters off the atoll about 1,700 kilometers south of Tokyo.
Sailing through the zone just outside the territorial waters of a coastal nation itself presents no issues under international law.
The Russian vessel had sailed south from the Tsushima Strait, which lies between the western main island of Kyushu and the Korean Peninsula, to the East China Sea at the end of March.
The ministry also said three Russian naval vessels transited the Tsushima Strait on Tuesday night to go up north to the Sea of Japan from the East China Sea.
The vessels -- consisting of a destroyer, frigate and supply ship -- had circumnavigated the Japanese archipelago, at one point entering the contiguous zone just outside territorial waters off the Senkaku Islands in the East China Sea.
China claims the Japanese-administered uninhabited islet group and calls it Diaoyu.
