East China Sea: Japan bolsters military near dispute Senkaku islands
A group of disputed islands, Uotsuri island (top), Minamikojima (bottom) and Kitakojima, known as Senkaku in Japan and Diaoyu in China is seen in the East China Sea, in this photo taken by Kyodo September 2012. Reuters

Japan is boosting its military presence in the East China Sea at a time when China's aggressive buildup in the East and South China Seas is causing increasing friction in the region.

The Japanese defence ministry has said it has doubled the number of F-15 fighter jets in the southern island of Okinawa, which is close to the disputed Senkaku islands in the East China Sea.

"This is a very front line of national defence," Deputy Defence Minister Kenji Wakamiya said at the Naha airbase in southern Okinawa, Jiji Press reported.

The airbase now has around 40 F-15 fighters.

The uninhabited islands became a point of contention between Tokyo and Beijing when China challenged Japan's authority over the islands in 1970.

The island chain is called Diaoyu by the Chinese. Beijing has increased patrol by coast guard vessels and surveillance aircraft in the region surrounding the island chain.

Japan had earlier announced the creation of a Ground Self-Defense Force base on its westernmost island, Yonaguni. Defense Minister Gen Nakatani said both measures were intended to strengthen the defence architecture in southwestern Japan in a "visible manner."

Japan's Self Defence Forces have said it recorded more than 500 incursions into its airspace between April and December last year. More than 65 percent of these were from Chinese aircraft.

Gas fields in disputed waters 

In 2004, China started developing four gas fields in East China Sea, following which both sides decided to set up a joint development region but a resolution has never been reached.

A major point in the increasing escalation in the conflict was in 2012 when Japan bought three of the islands from private owners and nationalised them. China responded by increasing its military presence in the region and more frequent violations of Japan's territorial waters.

In December 2015 Japan confirmed it was setting up anti-ship, anti-aircraft missile batteries on 200 islands in East China Sea and increasing the number of military personnel on these islands to around 10,000 over five years.