Philippine presidential spokesman Salvador Panelo said on Thursday that the passages were not “an act of friendship”.

“We express concern with that kind of incident ... I don’t think this is an act of friendship,” the Philippine Daily Inquirer quoted Panelo as saying. “It’s a violation of the United Nation Convention on the Law of the Sea (UNCLOS) since [the ships] passed through our exclusive economic zone.”

Analysts said there was debate over whether UNCLOS required prior notice for passage in international sea lanes such as the Sibutu Strait, but both China and the Philippines requested ships to do so.

Defiant Rodrigo Duterte shrugs off Reed Bank incident to defend his China policy in speech

The flare-up comes as Duterte is expected to make his fifth trip to Beijing later this month, when he is expected to raise long-standing issues over competing claims in the South China Sea and negotiations for a code of conduct in the waters.

Under Duterte, Manila has focused on improving economic ties with Beijing but critics say this has been at the expense of the Philippines’ maritime interests.

In 2016, the Permanent Court of Arbitration in The Hague ruled in favour of the Philippines, dismissing Beijing’s historical claims to a “nine-dash line” in the energy-rich South China Sea, which is claimed by several nations.

China refused to accept the decision and has expanded militarisation of its artificial islands in the waters, with China’s maritime militia involved in a growing number of confrontations.

In June near Reed Bank, a Chinese vessel sank a Filipino fishing boat with 22 people on board, raising pressure on Duterte to take a tougher line with Beijing.

Philippine Foreign Affairs Secretary Teodoro Locsin fired off a diplomatic protest after the incident. The Chinese embassy in Manila said the boats collided after the Chinese vessel was “suddenly besieged by seven or eight Filipino fishing boats”.

Aaron Rabena, research fellow at the Manila-based think tank Asia-Pacific Pathways to Progress, said Duterte would use his trip to not only further economic cooperation but also to advance joint exploration in the South China Sea or a fishing rights agreement at Scarborough Shoal.

But Rabena said many were watching to see if China would apologise for the Reed Bank incident or provide compensation to the Filipino fishermen.

“The expectations on the visit are high,” he said. “It is because of these incidents and the build-up in domestic pressure at home to take a more assertive stance vis-à-vis China that Duterte will bring up the arbitral ruling.

“At the same time, he will capitalise on the chance to bring to China’s attention reported incidents of unauthorised passage of Chinese warships and survey activities by Chinese research vessels within Philippine maritime waters and EEZ.”

Could Manila’s flip-flopping over Reed Bank make Beijing more aggressive?

Manila also lodged diplomatic protests in August over reports of two Chinese survey ships operating in the Philippines’ exclusive economic zone off its northern coast, and in July after Philippine authorities spotted some 100 Chinese fishing vessels around Thitu Island in the South China Sea.

But Hu Zhiyong, a researcher at the Institute for International Studies at the Shanghai Academy of Social Sciences, said Duterte’s visit indicated that ties between the two countries had entered a “strategic partnership” phase, and would not be affected by maritime disputes.

“President Duterte himself will not be willing to use these disputes to hurt the friendly relationship between China and the Philippines,” Hu said.

“So these small incidents before President Duterte’s trip to China will basically not become part of the discussion between the two leaders.”