Beijing-backed think-tank the South China Sea Probing Initiative (SSCBI) said three planes had been used to conduct secret surveillance flights over the disputed waters. A report from the group said the aircraft had taken off from bases in Okinawa in Japan and Manila in the Philippines and had carried 150 patrols in the East China, Yellow and South China seas since March.

It said US military chiefs had hired the civilian planes from private companies called Tenax Aerospace and Meta Special Aerospace.

Deploying the civilian spy planes would help boost the US military’s operational capabilities in the region.

The think tank described the covert surveillance operations as being “like a test to see Chinese reactions”.

Researchers from the group had previously accused the US military of flying military aircraft under civil aviation codes to avoid detection.

But they have now suggested deploying private planes in the region could lead to a reduction in tension and the risk of a major incident in the volatile region.

The report said: “Compared with the air reconnaissance capabilities of the US Navy and Air Force, the reconnaissance aircraft of private defence companies have greater flexibility in dealing with ‘grey area’ issues, reducing the diplomatic pressure caused by direct military confrontation.

“This also signals that the US will step up its presence in the Indo-Pacific region through a collaboration between the military, coastguard and private security sector.”

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China’s claims of sovereignty over virtually the entire South China Sea are contested by Taiwan, the Philippines, Brunei, Malaysia and Vietnam.

Beijing rejected a 2016 arbitration ruling invalidating most of its territorial claims in the South China Sea which resulted in an increased US military presence in the region.

Military flights over the contested waters have also intensified in recent months, according to  SSCBI.

US Army Major Randy Ready, a spokesman for the US Indo-Pacific Command, said: “American planes fly anywhere that it’s legal and continue their flights in Asia.”

“While the scope of our operations varies based on the current operating environment, the US has a persistent military presence and routinely operates throughout the Indo-Pacific, including the waters and airspace surrounding the East China Sea and the South China Sea.”

He called the air movement “a continued demonstration of our commitment to the region and our willingness to defend the freedoms enshrined in international law”.

US security analysists said and increase in surveillance flights over the seas near China reflected Washington’s drive to deter Chinese expansion and militarisation on the region.

Sean King, vice president of the Park Strategies political consultancy in New York said the operations “can be considered commensurate with the US State Department’s July policy statement that specific Chinese South China Sea claims are unlawful.”

He said Beijing would take no “actual action” against the US surveillance planes aside from making statements.

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Oh Ei Sun, senior fellow with the Singapore Institute of International Affairs, said officials in Washington would view the flights as routine.

He said: “What China would consider unusual might not be felt the same from the US side.”

Mr Oh said Southeast Asian countries that dispute China’s maritime claims “welcome” any increase in US military activity.

US Secretary of State Mike Pompeo has accused China’s governing Communist Party of “exploitation, corruption and coercion” in its treatment of other countries in the region.