It is reported by Kyodo with reference to the Ministry of Defense of Japan.
The Russian vessel was followed by a frigate of the Chinese Navy.
According to the agency, these are the same ships that were reported in the water area between the islands of Yonaguni and Iriomote on July 2.
Earlier, the Japanese authorities protested because of South Korea's exploration of the disputed islands.
On June 24, four patrol ships of the Chinese Maritime Police were spotted in the East China Sea near the Senkaku Islands.
Japan demanded that they leave the water area, as it considers it its territorial waters.