Chinese and Russian navy ships jointly sailed through a strait separating two Japanese islands on Monday.
That's according to Japan's government which said it was closely watching such activities.
The Tsugaru Strait separates the Sea of Japan from the Pacific and is considered international waters.
Japan's Deputy Chief Cabinet Secretary:
"It is the first time for Chinese and Russian naval ships to sail the Tsugaru Strait together, as you (media) have pointed out. The Japanese government has a strong interest in the activities of Chinese and Russian naval ships around Japan, so we are closely monitoring."
A Japanese Defence Ministry spokesperson said there had been no violation of Japanese territorial waters and no international rules had been broken.
Japan's ties with China have long been plagued by conflicting claims over a group of tiny East China Sea islets.
Tokyo has a territorial dispute with Moscow, as well.
Beijing and Moscow have sought closer ties in recent years as their relations with the West have soured.
Last week, they held joint joint naval drills in the Sea of Japan involving warships and multiple aircraft.