US Marines have conducted airfield - and island-seizure drills in the East and South China seas in what observers say is meant to remind Beijing of US military supremacy in the Asia-Pacific.

The 11-day naval drills were conducted near the Philippines and around the Japanese island of Okinawa by Okinawa-based US marine expeditionary units, the 31st Marine Expeditionary Unit said.

Observers said the operations were meant as a warning to Beijing that the US military could carry out amphibious campaigns far from home if Washington needed to intervene in territorial disputes between China and America's allies in the region.

The 31st Marine Expeditionary Unit and Amphibious Squadron 11 conducted joint weapons drills from their Wasp Amphibious Ready Group ships from August 9-19, the Okinawa-based marine unit said in a statement.

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The activity took place in the Philippine and East China seas and around an American naval base in Japan, it said.

The unit's Amphibious Reconnaissance Platoon also performed a reconnaissance and surveillance mission through a high-altitude, low-opening parachute jump onto Okinawa.

A tilt-rotor aircraft, which hovers like a helicopter but flies like an aeroplane, afterward sent a landing team from a Wasp ship more than 400km away to establish the arming and refuelling point. The team achieved its objective in just over one hour, the statement said.

"The speed with which the Marines were able to establish the forward arming and refuelling point demonstrates a capability that is critical to conducting expeditionary operations in a contested environment," the statement quoted Lieutenant Guirong Cai, a FARP officer-in-charge from the Marine Air Traffic Control Mobile Team, as saying.

"Their proficiency in swiftly setting up a refuelling point with 2.5 tonnes of fuel demonstrates the 31st MEU's ability to rapidly refuel and redeploy our air assets as the mission requires."

China has a territory dispute with Japan over Diaoyu Islands, known as the Senkakus in Japan, in the East China Sea, while both Beijing and Manila have put in claims on the Scarborough Shoal - also known in China as Huangyan Dao - in the South China Sea.

Adam Ni, a China specialist at Sydney's Macquarie University, said the drills near the Philippines and Okinawa showed that such a campaign would encompass a wide area, including the South and East China seas, where the US has joined other countries in the region to conduct freedom of navigation operations since 2015.

"It is a clear reminder to China of US military supremacy despite the narrowing of gaps in military capability in recent years," Ni said. "The message is that the US military can still take China-controlled South China Sea features in high-intensity conflict."

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The statement did not say whether the Philippine navy and Japanese maritime self-defence force took part in the drills. But Hong Kong-based military commentator Song Zhongping said the US government would call on its two allies to observe the exercises.

"Whether Washington will intervene in territorial disputes between China and the Philippines as well as China and Japan, the American [military] has used [these drills] to strengthen its island-capture and airfield seizure capabilities in unfamiliar waters and areas," Song said.

"To show its close relationship with and commitments to Manila and Tokyo, the Americans would invite the two allies to watch the drills. That could also be a good time to sell their amphibious warships and new model aircraft to Japan."

During the drills, 10 simulated casualties were treated by three medical technicians from the US Air Force's special operations group and given blood transfusions before being loaded onto a KC-130 transport aircraft for in-flight medical treatment en route to Marine Corps Air Station Futenma on Okinawa, the US Marines said.

- South China Morning Post