TOKYO -- Military helicopters took off from Chinese and Russian warships sailing near a Japanese island, according to the Ministry of Defense, days after a Sino-Russian naval passage through a chokepoint in Japan's north.

The latest incident took place around 4 a.m. Thursday about 50 km to 100 km southwest of Sumisu Island, part of the Izu archipelago stretching south of Tokyo off Japan's Pacific side. Two helicopters flew from frigates operated by China and Russia.

Japan scrambled Air Self-Defense Force fighter jets in response. The two helicopters returned to their respective frigates without violating Japanese airspace. Chinese and Russian warships did not enter Japanese territorial waters.

The Japanese ministry will strengthen vigilance as it rushes to analyze the intent behind the incident.

Ten Chinese and Russian warships sailed Monday through the Tsugaru Strait, a narrow chokepoint between Japan's main island of Honshu and the northern island of Hokkaido. They were apparently part of a joint exercise in the Sea of Japan.