TOKYO (AP): Japan expressed concern on Monday after China and Russia held a joint naval exercise in which 10 of their warships passed through international waterways between its northern and southern islands.

The Chinese and Russian vessels were first spotted last Monday passing through the Tsugaru Strait between Japan's main island of Honshu and the northern island of Hokkaido. They passed through the Osumi Strait off the southern island of Kyushu on Thursday, completing a near circle around Japan's main island before heading back toward China.

Yoshihiko Isozaki, the deputy chief Cabinet secretary, said at a regular briefing that it was the first time Japan has confirmed that Chinese and Russian naval vessels passed jointly through the Tsugaru and Osumi Straits and in waters near the Izu islands off the main island's south-eastern coast.

The straits are considered international waters. Japanese officials said the Chinese and Russian ships did not enter Japanese territorial waters.

Japan considers China's increasingly assertive maritime activities as a regional security threat. The two countries are also locked in a dispute over Japanese-controlled East China Sea islands that China claims as part of its territory.

Japan also has a territorial row with Russia over a separate group of islands north of Hokkaido that has prevented them from signing a peace treaty, formally ending their hostility in World War II.

Isozaki said officials also confirmed that the Russian warships sailed through the Tsushima Strait off the main island's north-western coast after separating from the Chinese vessels. He said Japan's Self Defence Force confirmed helicopters flying off both the Chinese and Russian vessels and that Japan responded by scrambling its fighter jets.

The Japanese Government is paying close attention with high interest to their activities, Isozaki said. He said Tokyo will continue its vigilance and surveillance in waters and airspace around Japan.