Japan has doubled the number of F-15 fighter jets deployed on the southern island of Okinawa, near disputed islands in the East China Sea, the defence ministry said Sunday.

Japan's Air Self-Defence Force now has about 40 F-15s on Okinawa's Naha base, according to the defence ministry.

The move comes as Japan and China have routinely clashed over ownership of the Tokyo-controlled Senkaku Islands in the East China Sea.

Japan administers the uninhabited islands under that name but China also claims them and calls them the Diaoyus.

Chinese ships -- mostly coast guard vessels -- and aircraft have approached them to back up Beijing's claims and test Japan's response.

"This is a very front line of national defence," said Deputy Defence Minister Kenji Wakamiya, quoted by Jiji Press, at the Sunday ceremony at the Naha base to mark the creation of a new unit composed of the extra jets.

The defence ministry moved about 20 F-15s from the Tsuiki base in the southwestern island of Kyushu to Naha, the defence ministry said.