PRESIDENT Rodrigo R. Duterte denounced last week’s aggression by Chinese Coast Guard vessels against two Philippine-flagged boats during a leaders’ summit on Monday of China and members of the Association of Southeast Asian Nations (ASEAN).

“We abhor the recent event in the Ayungin Shoal and view with grave concern other similar developments,” Mr. Duterte said.

“This does not speak well of the relations between our nations and our partnership.”

Mr. Duterte made the statement during the virtual ASEAN-China special summit, which was held a week after Chinese Coast Guard ships illegally blocked and discharged water cannons on boats that were carrying supplies to a military post on a Philippine-claimed reef in the South China Sea.

The Philippine leader said the South China Sea dispute is a strategic challenge that cannot be solved by force.

He called on stakeholders to exercise self-restraint, avoid the escalation of tensions, and work to resolve the dispute peacefully in accordance with international laws.

The presidential palace said Mr. Duterte affirmed a United Nations-backed arbitral award that invalidated China’s claim to more than 80% of the sea based on a 1940s map and told Beijing to remain committed to the conclusion of an effective and substantive code of conduct in the disputed waterway.

“There is simply no other way out of this colossal problem but the rule of law,” he said, less than a year before his six-year term ends.

The presidential palace, meanwhile, said Mr. Duterte praised China for its efforts to help countries affected by the pandemic.

The Philippines, China, Vietnam, Malaysia, Brunei and Taiwan claim parts of the South China Sea.

The US, which is not a claimant, has accused China of flexing its military muscle and restricting freedom of navigation there. It has pledged to provide security support to its Asian allies, including the Philippines.

Last week, the US called the Chinese attacks “dangerous, provocative, and unjustified,” warning that it would invoke its defense pact with the Philippines in case of an armed attack on Filipino-manned vessels.

Foreign Affairs Secretary Teodoro “Teddy Boy” L. Locsin, Jr., who also participated in the summit, said in October that the Philippines was backing a defense pact that allows Australia to build nuclear-powered submarines using technology that the United States had only previously shared with Britain, saying it could keep the balance of power in the Indo-Pacific region.

BBC News reported last month that a US nuclear submarine had hit a mystery object in the sea.

Tan Kefei, a spokesman for China’s defense ministry, has demanded a clear explanation of the incident, the South China Morning Post reported.

The South China Sea, which is important for the regional ambitions of China, is a source of tension in the Indo-Pacific as the US and other Western countries continue to assert freedom of navigation.

During Monday’s summit, Chinese President Xi Jinping told leaders of the 10-member ASEAN that Beijing would not “bully” its smaller regional neighbors amid rising tension over the South China Sea.

“China was, is, and will always be a good neighbor, good friend, and good partner of ASEAN,” state media quoted Mr. Xi as saying.

China would never seek hegemony nor take advantage of its size to coerce smaller countries, and would work with ASEAN to eliminate “interference,” Mr. Xi said.

Mr. Duterte led a foreign policy pivot to China away from the US when he took office in 2016. Less than a year before he steps down, Mr. Duterte has changed his tone toward the US.

He has thanked US President Joseph R. Biden for donating coronavirus vaccines to the Philippines. He also restored a visiting forces agreement after suspending it for months. The two nations are set to hold more than 300 joint defense activities next year. — Kyle Aristophere T. Atienza with Reuters