The Philippines may have underestimated China's ability to hit back in the South China Sea, a Chinese maritime expert warned on Wednesday.

Addressing a closed-door meeting on the China-Philippines relationship, Wu Shicun, chairman of the Huayang Centre for Maritime Cooperation and Ocean Governance, also cautioned that Manila's "unprecedented" security cooperation with the US would risk not just ties with China but also the Philippines' own interests.

"An important prerequisite for stable China-Philippine relations in the future is that the US-Philippine alliance and security cooperation must not target China," Wu said, according to a transcript published by the Hainan-based think tank.

"But if their cooperation in, for example, the use of military bases, joint exercises, information gathering and logistical supplies infringe the core and important interests of China such as Taiwan, the South China Sea and ...[China's] national security, then not only will China-Philippines relations bear the brunt, and the peace and stability of the South China Sea become untenable, the Philippines' own interests will also be undermined."

Ties between Beijing and Manila have been strained over repeated face-offs between Chinese and Philippine ships in the South China Sea, including collisions that have raised fears of the waterway becoming a flashpoint.

Beijing and Manila have accused each other of provocation.

In an interview after arriving in Japan last Friday, Philippine President Ferdinand Marcos Jnr, who has strengthened Manila's alliance with Washington, said his country needed a "paradigm shift" in its approach to the South China Sea because diplomatic efforts with Beijing were heading "in a poor direction".

Marcos said the Philippines would work with partners in the Indo-Pacific region to come up with a joint position on their responsibilities in the waters.

During the meeting on Wednesday, Wu, the former president of the government-backed National Institute for South China Sea Studies, said US support was partly to blame for recent "risky behaviours" by the Philippines.

Despite blockades by Beijing, Manila has sent regular resupply missions to shore up a World War II-era navy ship that was deliberately grounded in 1999 as an outpost at the Second Thomas Shoal.

"[The US support was also] linked to the underestimation made by the Philippines' strategic community and policymakers of China's likely resolve and ability to counterbalance it," he added.

Beijing has long labelled the US as an "external force" escalating tensions in the strategically important waterway that connects Northeast and Southeast Asia, and the Pacific and the Indian Ocean.

It was frustrated when Manila announced in April that it would allow US troops access to four more military bases, including one about 400km (250 miles) from Taiwan and another one that is only 200km from a Chinese base on Mischief Reef in the Spratly Islands.

In another move that may upset Beijing, Japan, another US treaty ally, announced on Wednesday that it had formally handed over an air surveillance radar system to the Philippines.

The system can detect approaching fighter jets and missiles and help bolster the Southeast Asian country's defences, according to Philippine Defence Secretary Gilberto Teodoro.

Wu said that a stronger US-Philippine alliance would bring more uncertainties to Manila's ties with Beijing.

This is especially so given the US has steadily increased its military presence around China, including sailing of naval vessels and surveillance planes in the South China Sea and through the Taiwan Strait, according to Wu.

Marcos has said the US cannot use its military bases for offensives against China but Wu warned that such situations would be "out of the Philippines' control".

"A likely consequence is that the Philippines not only becomes a casualty of the US-China military game in the South China Sea, but also becomes militarily involved in the US-China conflict in the Taiwan Strait," Wu said.

The two sides should find a way out of the "South China Sea dilemma" to avoid spillover damage on bilateral cooperation, Wu said.

And the only way to do that, according to Wu, is to set aside a ruling from a tribunal in The Hague that dismissed much of Beijing's claim to the disputed waters.

Chinese structures and buildings are seen on a man-made island on Mischief Reef in March. Photo: AP alt=Chinese structures and buildings are seen on a man-made island on Mischief Reef in March. Photo: AP>

"The only way is to set aside the South China Sea tribunal ruling and not take it as an additional condition when dealing with bilateral relations and the South China Sea disputes," he said.

"Otherwise ... the South China Sea dilemma will never be resolved and the comprehensive cooperation in trade, culture ... tourism and investment would inevitably be interrupted."

He also said the two sides should resume the hotline between their respective coastguards, which started in January but was suspended in August after Philippine Coast Guard spokesman Jay Tarriela complained that the line "never really gave us a positive chance to talk".

Wu said the hotlines as well as the joint committee between the two coastguards since 2017 had "played some role" in managing disputes at the sea, including in the waters around the Scarborough Shoal, the Second Thomas Shoal and Sand Cay, a China-controlled high-tide sandbar located about two nautical miles from the Philippines' Pagasa Island.

"There is no reason for us to set aside these effective mechanisms ... and allow conflicts to widen, conflicts to escalate and crises to spiral out of control, and watch relations between our two countries reach the point of no return," Wu said.

China warns Philippines against South China Sea 'miscalculation'

Chinese Foreign Minister Wang Yi warned the Philippines that any miscalculation in their escalating dispute in the South China Sea would bring a resolute response, and urged dialogue to address "serious difficulties" between the two neighbours.

Beijing and Manila have traded sharp accusations in recent months over a succession of run-ins in the South China Sea, including charges that China rammed a ship earlier this month carrying the Philippine armed forces chief of staff.

China in turn has accused the Philippines of trespassing on its territory.

The souring of relations coincides with Manila's moves to boost military ties with Japan and the United States, its former colonial power and defence ally of seven decades.

"China-Philippines relations are at a crossroads," Wang told his Philippine counterpart Enrique Manalo in a call on Wednesday, according to a Chinese foreign ministry statement. A ministry spokesperson said Manalo had requested the call.

If the Philippines misjudges or colludes with "ill-intentioned" external forces, China would defend its rights and respond resolutely, Wang was quoted saying. The statement did not elaborate on what actions it might take.

Wang's remarks could intensify a dispute that has simmered for years, with the Philippines pushing back at what it sees as a Chinese campaign to prevent it from accessing fossil fuel and fisheries resources in its exclusive economic zone (EEZ).

An escalation towards an armed confrontation, while unlikely, would be a significant raising of the stakes, with the United States bound by a 1951 treaty to defend the Philippines should it come under attack, including in the South China Sea.

Philippine President Ferdinand Marcos Jr, in a speech before troops on Thursday, said the country remained committed to bolstering its armed forces and its existing alliances, while citing the incident involving the armed forces chief as worrisome.

"You have become crucial as in the past years the Philippines has found itself in the middle of geopolitical developments and tensions that could potentially cause regional insecurity," he told the gathering at military headquarters in Manila.

LINES IN THE SOUTH CHINA SEA

China claims almost the entire South China Sea via a so-called nine dash line that overlaps with the EEZs of the Philippines, Brunei, Malaysia, Taiwan, Vietnam and Indonesia.

An international arbitration tribunal in 2016 invalidated China's claim in a ruling on a case brought by the Philippines, which Beijing did not recognise.

China has instead doubled down, maintaining a heavy coast guard presence throughout the South China Sea, including around militarised manmade islands it built upon reefs in disputed waters, some with missile systems.

Both sides have, at the same time, called for dialogue.

Manalo said in a statement that he had a frank and candid exchange with Wang where both "noted the importance of dialogue".

A Chinese foreign ministry spokesman said on Thursday: "China's position remains unchanged that disputes should be properly managed through dialogue and consultation."

Marcos has strengthened ties with the United States, including expanding U.S. access to his military bases while seeking assurances on the extent to which Washington will defend his country from attack - moves that have irked China and emboldened Manila's defence top brass.

Philippines Defence Secretary Gilberto Teodoro said on Thursday his country was also likely to hold multilateral patrols next year in the West Philippine Sea - the term it uses for its 200-mile EEZ in the South China Sea.

A day earlier, Teodoro rebuked China and said "no country in the world" supports its maritime claim. The United States and other western powers have condemned China's coastguard for confronting and blocking Philippine vessels in Manila's EEZ.