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The US Navy P-8A Poseidon jet was flying at 16,500 feet to get a view of low-lying coral reefs that have been turned into garrisons with five-storey buildings, large radar installations, power plants and runways sturdy enough to carry large military aircraft. During the flight that was giving journalists from CNN a rare look at the islands, the crew was warned six times by the Chinese military to get out of their territory. A voice said: “US military aircraft, this is China ... leave immediately and keep out to avoid any misunderstanding.” Each time the aircraft was challenged by the Chinese military, the US Navy crew's response was the same.

The response was: "I am a sovereign immune United States naval aircraft conducting lawful military activities beyond the national airspace of any coastal state. "In exercising these rights as guaranteed by international law, I am operating with due regard for the rights and duties of all states." CNN was granted the chance to see how the Chinese government is rapidly expanding its militarisation efforts from a US reconnaissance plane when the harsh threat was issued. The US Navy jet had flown over four key artificial islands in the Spratly chain where China has built up fortifications: Subi Reef, Fiery Cross Reef, Johnson Reef and Mischief Reef.

China has threatened the US

The Navy jet's sensors picked up on 86 vessels on the Subi Reef

Before the crew were warned, the jet's sensors picked on 86 vessels on the Subi Reef that included Chinese coast guard ships. On the Fiery Cross Reefs, rows of hangars stood alongside a lengthy runway. LT. Lauren Callen who lead the air combat crew on the Navy flight said: "It was surprising to see airports in the middle of the ocean." Beijing has said it is necessary for the Asian powerhouse to keep growing its military presence in the South China Sea in order to protect its sovereignty. China also blames Washington and its allies for tensions in the region.

The crew also flew over the Mischief Reef in the South China Sea

On the Fiery Cross Reefs rows of hangars were spotted on a runway

It says that the regular US Navy patrols and flyovers of the South China Sea are efforts for the US to provoke China and therefore the country is justified in increasing its military presence. Chinese Foreign Ministry spokeswoman Hua Chunying said: "By playing up the so-called China's militarisation in the South China Sea, certain people in the US are staging a farce of a thief crying 'stop thief'. "It is self-evident to a keener eye that who is militarising the South China Sea." Parts of the South China Sea are claimed by Vietnam, the Philippines, Taiwan, Indonesia, Malaysia and Brunei.

The South China Sea a highly-disputed part of the world

The sea spans 3.6million square-kilometres and Beijing's claims extend more than one thousand kilometres from its southernmost province, which is almost the entirety of the waters. The United Nations has estimated that a third of global shipping passes through the waters. The South China Sea is also believed to be rich in oil and natural gas reserves that have yet to be explored. China reinforced its claims by constructing the artificial islands which are militarised with airfields and radar equipment.

The US Navy also flew over Johnson Reef

In April the military placed missiles on the Spratly island chain during naval exercises. This broke a promise by President Xi Jinping to then US-President Barack Obama in 2015 that the Chinese government would not militarise the artificial islands. Experts claim that time is running out to challenge China's claims in the South China Sea.