BEIJING: More than 40 Chinese military aircraft have traversed a strait near Japan, China's defence ministry said Monday, after Tokyo announced it may patrol with the US in the contested South China Sea region.

China's Air Force on Sunday sent 40 planes over the Miyako Strait, between Japan's Miyako and Okinawa Islands, to carry out training in the West Pacific, according to a statement on the defence ministry's website.

The planes -- Sukhoi Su-30 fighters, bombers and refuelling aircraft -- made the flights without violating Japan's airspace.

The drill is aimed at “testing far sea combat capabilities”, the statement said, and follows China's first military flight over the Miyako strait last year.

The move comes after Japanese defence minister Tomomi Inada said earlier this month that Tokyo would increase its engagement in the South China Sea through joint training cruises with the US Navy, exercises with regional navies and capacity-building assistance to coastal nations.

Beijing asserts sovereignty over almost all of the strategically vital waters, dismissing rival claims from its Southeast Asian neighbours.

In recent months, Japanese Prime Minister Shinzo Abe has criticised China for rejecting a July ruling by an international tribunal that said Beijing's extensive claims to the waters had no legal basis.

Tokyo, a key US ally, is also boosting defence ties with other countries in the disputed region, with the two Asian powerhouses already at loggerheads over a longstanding territorial row in the East China Sea. That dispute relates to uninhabited islets controlled by Japan in the East China Sea, known as the Senkakus in Japanese and the Diaoyus in Chinese.

The Chinese defence ministry said in the statement that it had also mobilised an unspecified number of bombers and fighters to patrol in the East China Sea air defence identification zone (ADIZ).

Beijing sparked alarm after it unilaterally established the ADIZ in 2013, demanding all aircraft submit flight plans when traversing the zone, which covers islands disputed with Tokyo and also claimed by Taipei.

“Normalising far sea drills out in the West Pacific and patrols in the East China Sea ADIZ is based on the need for China's Air Force to protect national sovereignty and security and ensure peaceful development,” Air Force spokesperson Shen Jinke said in the statement.

The Chinese military has been monitoring and identifying foreign army planes that entered the ADIZ and “took measures according to different air threats” since the zone was set up three years ago, the statement added.

Japan vows to closely watch possible China warplane activity

Meanwhile, Japan's top government spokesman has warned China against expanding its military activity in the skies over disputed East China Sea islands after Chinese warplanes flew near the area on Sunday.

Chief Cabinet Secretary Yoshihide Suga said Monday that Japan scrambled at least one fighter jet after the planes passed over the Miyako Strait, east of the Japan-controlled Senkaku islands, which China also claims and calls the Diaoyu.

Suga said the aircraft, believed to be two fighter jets, four bombers and two reconnaissance aircraft, did not violate Japanese airspace during a “long-distance roundtrip flight.”

He added that Japan would respond firmly to any violation of Japanese air space in the future.