DigitalGlobe Satellite imagery has indicated Chinese aircraft have been deployed to the South China Sea

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Photos taken by commercial firm DigitalGlobe in May show four submarine-hunters, three Harbin BZK-005 recon unmanned aerial vehicles and two KJ-500 early warning jets parked at Lingshui Air Base on Hainan Island. The Y-8X aircraft were put into service in 2015 but have not been documented at the Lingshui Air Base until now, according to Defense News. ​The submarine-hunter is the first combat-ready maritime patrol plane to be commissioned by China’s People’s Liberation Army Navy (PLA-N).

The Lingshui Air Base gained huge attention in 2001 when a US Navy EP-3 intelligence, surveillance and reconnaissance (ISR) aircraft crashed with a Chinese aircraft in midair, killing the Chinese pilot involved. The US Navy aircraft was reportedly forced to send out a “mayday” signal and make an emergency landing at the Linshui Air Base. Earlier this month, the US issued a stark warning that it would not accept China’s militarisation of man-made islands in the South China Sea.

GETTY China's territorial claims in the South China Sea are contested by other countries

Discover the disputed islands of the South China Sea Wed, May 10, 2017 Among the islands at the heart of territorial disputes in the South China Sea. Play slideshow Getty Images 1 of 6 Subi Reef at Spratly Islands: Subi Reef has transformed from a single building and cranes on an artificial sand bank to what looks look a forward operating base with its own town.

Speaking at a security conference in Singapore, US Defence Secretary James Mattis said such moves undermined regional stability and would not be tolerated. China's territorial claims in the South China Sea – through which about £3.9trillion in ship-borne trade passes each year – are contested by Brunei, Malaysia, the Philippines, Taiwan and Vietnam. President Donald Trump and other senior US officials have repeatedly stated that they would protect its interests in the South China Sea - a key shipping route.

GETTY James Mattis has warned against Chinese militarisation of man-made islands in the South China Sea