For the first time, China has publicised what it claims is an unwritten 2016 agreement with the Philippines over access to South China Sea islands.

The move threatens to further raise tensions in the disputed waterway, through which much of the world's trade passes and which China claims virtually in its entirety.

A statement from the Chinese Embassy in Manila said the “temporary special arrangement” agreed to during a visit to Beijing by former president Rodrigo Duterte allowed small scale fishing around the islands but restricted access by military, coast guard and other official planes and ships to the 12 nautical mile (22 km) limit of territorial waters.

The Philippines respected the agreement over the past seven years but has since reneged on it to “fulfil its own political agenda”, forcing China to take action, the statement said.

“This is the basic reason for the ceaseless disputes at sea between China and the Philippines over the past year and more,” said the statement posted to the embassy's website Thursday, referring to the actions of the Philippines.

President Ferdinand Marcos Jr and Duterte have denied forging any agreements that would have supposedly surrendered Philippine sovereignty or sovereign rights to China. Any such action, if proven, would be an impeachable offense under the country's 1987 Constitution.

However, after his visit to Beijing, Duterte hinted at such an agreement without offering details, said Collin Koh, senior fellow at the S Rajaratnam School of International Studies based in Nanyang Technological University, Singapore and an expert on naval affairs in the Indo-Pacific region, particularly Southeast Asia.

“He boasted then that he not only got Chinese investment and trade pledges, but also that he secured Philippine fishermen access to Scarborough Shoal,” Koh said, referring to one of the maritime features in dispute.

Beijing's deliberate wording in the statement “is noteworthy in showing that Beijing has no official document to prove its case and thus could only rely mainly on Duterte's verbal claim,” Koh said.

Marcos, who took office in June 2022, told reporters last month that China has insisted that there was such a secret agreement but said he was not aware of any.

“The Chinese are insisting that there is a secret agreement and, perhaps, there is, and, I said I didn't, I don't know anything about the secret agreement,” said Marcos, who has drawn the Philippines closer to its treaty partner the U.S. “Should there be such a secret agreement, I am now rescinding it.”

Duterte, who nurtured cozy relations with Chinese President Xi Jinping during his six-year presidency while openly being hostile to the United States for its strong criticism of his deadly campaign against illegal drugs.

While he took an almost virulently anti-American stance during his 2016 visit to Washington's chief rival, he has said he also did not enter into any agreement with Beijing that would have compromised Philippine territory. He acknowledged, however, that he and Xi agreed to maintain “the status quo” in the disputed waters to avoid war.

“Aside from the fact of having a handshake with President Xi Jinping, the only thing I remember was that status quo, that's the word. There would be no contact, no movement, no armed patrols there, as is where is, so there won't be any confrontation,” Duterte said.

Asked if he agreed that the Philippines would not bring construction materials to strengthen a Philippine military ship outpost at the Second Thomas Shoal, Duterte said that was part of maintaining the status quo but added there was no written agreement.

“That's what I remember. If it were a gentleman's agreement, it would always have been an agreement to keep the peace in the South China Sea,” Duterte said.

House Speaker Ferdinand Martin Romualdez, Marcos's cousin and political ally, has ordered an investigation into what some are calling a “gentleman's agreement”.

China has also claimed that Philippine officials have promised to tow away the navy ship that was deliberately grounded in the shallows of the Second Thomas Shoal in 1999 to serve as Manila's territorial outpost. Philippine officials under Marcos say they were not aware of any such agreement and would not remove the now dilapidated and rust-encrusted warship manned by a small contingent of Filipino sailors and marines.

China has long accused Manila of “violating its commitments” and “acting illegally” in the South China Sea, without being explicit.

Apart from China and the Philippines, Vietnam, Malaysia, Taiwan and Brunei also have overlapping claims in the sea that is rich in fishing stocks, gas and oil. Beijing has refused to recognise a 2016 international arbitration ruling by a UN-affiliated court in the Hauge that invalidated its expansive claims on historical grounds.

Skirmishes between Beijing and Manila have flared since last year, with massive Chinese coast guard cutters firing high-pressure water cannons at Philippine patrol vessels, most recently off Scarborough Shoal late last month, damaging both. They have also accused each other of dangerous maneuvering, leading to minor scrapes.

The US lays no claims to the South China Sea, but has deployed Navy ships and fighter jets in what it calls freedom of navigation operations that have challenged China's claims.

The US has warned repeatedly that it's obligated to defend the Philippines — its oldest treaty ally in Asia — if Filipino forces, ships or aircraft come under an armed attack, including in the South China Sea.