Japan scrambled jets after 11 Chinese military planes flew near southern Japanese islands during what Beijing said was a drill to improve its long-range combat abilities, reports said Saturday.

The planes -- eight bombers, two intelligence gathering planes and one early-warning aircraft -- flew near Miyako and Okinawa on Friday without violating Japan's airspace, the Japanese defence ministry said in a statement released on Friday.

Some of them flew between the two islands while others made flights close to neighbouring islands, the ministry said.

A Chinese air force spokesman said several types of planes, including H-6K bombers, were involved in Friday's drill over the western Pacific, China's Xinhua news agency reported.

Shen Jinke said such open sea exercises had improved the force's long-distance combat abilities, according to Xinhua.

While there were no further comments from the Japanese ministry, the Yomiuri Shimbun reported that it was "unusual" for China to dispatch such a large fleet close to Japan's airspace and the ministry was analysing the purpose of the mission.

Japan scrambles jets hundreds of times a year to defend its airspace, both against Russia and these days also against Chinese aircraft.

Beijing has warned this is heightening tensions between the two Asian powerhouses, which are already at loggerheads over a longstanding territorial row in the East China Sea and Japanese military aggression in the first half of the 20th century.

The move comes with tensions running high in the South China Sea after a US warship sailed close to at least one land formation claimed by China, which has rattled its neighbours with its increasingly assertive stance in territorial disputes.

China transformed reefs in the region into small islands capable of supporting military facilities, a move the US says threatens freedom of navigation in a region through which one-third of the world's oil passes.

China insists on sovereignty over virtually all the resource-endowed South China Sea, which is also claimed in part by a handful of other countries.

Washington has repeatedly said it does not recognise the Chinese claims.