Japan's Maritime Self-Defense Force has confirmed that five Russian navy ships sailed into the Sea of Japan, after passing through the Tsushima Strait on Tuesday morning.
The strait is an international waterway where foreign ships including warships, are allowed to navigate.
The five vessels were spotted reaching the East China Sea on Sunday, after passing through waters between two islands of Okinawa Prefecture in southwestern Japan.
Last Wednesday, the ships had been spotted off Cape Erimo in northernmost prefecture of Hokkaido. They then sailed south to waters off Chiba Prefecture neighboring Tokyo and the Izu Islands south of the capital.
Japanese Defense Ministry sources say the vessels are believed to have participated in a large-scale exercise in the Pacific Ocean. The Russian defense ministry had said it would conduct a drill this month involving more than 40 vessels.
The MSDF also spotted three Chinese naval vessels sailing waters between two Izu islands, heading west early on Tuesday.
The three ships had been seen separating into two groups to pass through the Soya and Tsugaru straits, heading east. They then sailed south off Chiba, like the five Russian vessels.
Both straits are international waterways just like the Tsushima straits.
The fleets of the two countries have been sailing around the Japanese archipelago, taking similar routes.
The Japanese Defense Ministry is analyzing the purposes of their navigation, while monitoring their activities.