Japan says it has doubled the number of F-15 warplanes stationed in Okinawa Island close to the disputed East China Sea islands.

Japan’s Defense Ministry said on Sunday that it has moved about 20 F-15 fighter jets from the Tsuiki airbase in the island of Kyushu to Okinawa’s Naha airbase.

“This is a very front line of national defense,” said Japan’s Deputy Defense Minister Kenji Wakamiya said during the ceremony to mark the creation of a new Air Force unit at the Naha base.

The deployment brings to 40 the total number of F-15s on Okinawa’s Naha base.

Japan and China have also been at odds over the sovereignty of a chain of islands in the East China Sea, known as Senkaku in Japan and as Diaoyus in China.

The uninhabited yet strategically-important island group has been under Japanese administrative control since the reversion of Okinawa to Japan from US administrative rule in 1972.

China maintains that the islands are inherent part of its territory and that it has indisputable sovereignty over them, while the Japanese government regards the islands as a part of its Ishigaki Island.