An AFP photographer on the flight described seeing the helicopter tail the plane before drawing near the left wing, close enough to see personnel aboard filming them.

The People's Liberation Army Navy (PLAN) helicopter had been "as close as three metres" to the fisheries bureau's Cessna, the coast guard said in a statement. "This reckless action posed a serious risk to the safety of the pilots and passengers."

The plane had been flying about 213 metres (700 feet) above the water on a mission to observe Chinese vessels around the shoal.

The Scarborough Shoal -- a triangular chain of reefs and rocks in the South China Sea -- has been a flashpoint between the countries since China seized it from the Philippines in 2012.

China claims the South China Sea almost in its entirety despite an international ruling in 2016 concluding this has no legal basis.

Coast guard spokesman Commodore Jay Tarriela told reporters Tuesday's incident was the first time a PLAN helicopter had been used against a Philippine patrol plane in such a manner.

"Less than 10 feet. So that's very dangerous. It can definitely affect the stability of the aircraft," Tarriela told a briefing after the incident.

Asked if the encounter marked an escalation, Tarriela said he believed China was taking a "calibrated approach" to such interactions, while reiterating President Ferdinand Marcos's declaration that the country's mutual defence treaty with the United States could be invoked if a death were to result.

Tian Junli, spokesperson for China's Southern Theater Command and senior colonel of the Air Force, said the Philippine aircraft had "illegally intruded into Chinese airspace over Huangyan Island", using the Chinese name for the shoal.

He said naval and air units were deployed "to track, monitor, warn, and expel the aircraft in accordance with the law and regulations", adding that the "actions of the Philippine side seriously violated China's sovereignty".

The Philippine government said later on Tuesday evening that it would file a formal diplomatic protest over the incident.

- Flares, collisions -

The incident comes less than a week after Australia rebuked Beijing for "unsafe" military conduct, accusing a Chinese fighter jet of releasing flares within 30 metres of a surveillance plane patrolling above the South China Sea.

A Chinese foreign ministry spokesman said at the time the Australian plane had "deliberately intruded into the airspace around China's Xisha Islands", Beijing's name for the Paracel Islands, which Vietnam and Taiwan also claim.

The Scarborough Shoal has been the site of repeated confrontations as Manila has resupplied Filipino fishermen in the area. It lies 240 kilometres (150 miles) west of the Philippines' main island of Luzon and nearly 900 kilometres (560 miles) from Hainan, the nearest major Chinese land mass.

In December, the Philippines said the Chinese coast guard used water cannon and "sideswiped" a government fisheries department vessel.

Manila released a video appearing to show a Chinese coast guard ship directing a torrent of water at the BRP Datu Pagbuaya.

Other footage apparently taken from the Philippine ship showed its crew shouting "Collision! Collision!" as the much larger Chinese vessel nears its right-hand side before crashing into it.

Manila and treaty ally Washington have deepened their defence cooperation since Marcos took office in 2022 and began pushing back against China's claims to the South China Sea.

The Philippines said in December it hoped to acquire the US Typhon missile system as part of a push to secure its maritime interests.

The mid-range missile system, deployed in 2024 for annual joint military exercises, has a range of 480 kilometres (300 miles), although a longer-range version is in development.

China has warned that acquiring the system risks triggering an "arms race".