Japan has heaped pressure on China by claiming it has attempted “to restrict the freedom of overflight” in the area. It referred to a dismissal of Beijing’s claim to the waterway by an international tribunal in The Hague in 2016.
Speaking at the time, Chinese president Xi Jinping said the superpower’s “territorial sovereignty and marine rights” in the seas would not be affected as he rejected the ruling.
Donald Trump consistently challenged China during his term in office, flaring tensions in the disputed waters.
Japan’s involvement, revealed by the South China Morning Post, was highlighted following a message to the United Nations on Tuesday which claimed Beijing was not meeting conditions set out in the UN Convention on the Law of the Sea.
The note, addressed to UN secretary general Antonio Guterres, referred to The Hague ruling.
It said: “China has not accepted this  award, and has asserted that it has ‘sovereignty’ in sea and airspace surrounding and above those maritime features found to be low-tide elevations.
“As a matter of fact, China protests the overflight of Japanese aircraft in the surrounding Mischief Reef and attempts to restrict the freedom of overflight in the South China Sea.”
The Trump administration has supplied multi billion arms packages to the island of Taiwan, claimed by China, which is also developing its own weapons to fight back against Beijing.
The US has also announced plans to bolster its forces by integrating its Navy, Marine Corps and Coast Guard for deployment in the region.
Chinese warplanes made at least 380 incursions into Taiwan’s air defence identification zone (ADIZ) in 2020, according to Shih Shun-wen, a Taiwanese defence ministry spokesman.
Mr Trump’s Secretary of State Mike Pompeo said last week: “The United States stands with Southeast Asian claimant states seeking to defend their sovereign rights and interests, consistent with international law. We will continue to act until we see Beijing cease its coercive behaviour in the South China Sea.”