Japan's Defense Ministry says it detected a total of 10 aircraft of the Chinese and Russian militaries flying near the country on Friday, with some conducting a joint flight.

The ministry said two Chinese and two Russian bombers staged a joint long-distance flight over the Sea of Japan and the East China Sea on Friday afternoon, and that one Russian fighter jet joined them.

It also said a total of five Chinese military planes, including fighter and intelligence-gathering aircraft, flew over the East China Sea and the Sea of Japan before or after the joint flight.

Japan's Air Self-Defense Force scrambled fighter jets to respond to the series of flights. Officials say there was no violation of Japan's airspace.

The Defense Ministry said it has conveyed grave concern to China and Russia through diplomatic channels respectively, stating the flights were clearly intended as demonstrative action toward Japan.

This is the first time since December last year, and the ninth in total, that the ministry has announced joint flights of Chinese and Russian military planes near Japan.

Officials remain on alert as Beijing and Moscow are apparently stepping up military cooperation.