BEIJING - China warned Thursday of a “decisive response” to provocations in the South China Sea, as it faced mounting pressure to accept an international tribunal’s ruling against its claims to most of the strategically vital waters.

The Philippines, which launched the legal challenge, called for China to respect the decision and defied Chinese objections by saying it would raise the issue at a summit of Asian and European leaders in Mongolia starting on Friday.

China, which had already vowed to ignore Tuesday’s verdict by the UN-backed tribunal in The Hague, responded with another firm warning that it was in no mood to back down.

“If anyone wants to take any provocative action against China’s security interests based on the award, China will make a decisive response,” foreign ministry spokesman Lu Kang said.

Southeast Asia will not issue a statement on the rejection of Chinese territorial claims in the South China Sea by an international tribunal, regional diplomats said Thursday, blaming the no-comment on pressure by Beijing.

The Association of Southeast Asian Nations (ASEAN) had weighed whether to speak out on Tuesday’s ruling by the Permanent Court of Arbitration in The Hague, said Southeast Asian diplomats with knowledge of the matter.

But 10-member ASEAN, whose unity has been increasingly strained in the face of Chinese expansionism, could not find common ground, they said.

China claims nearly all of the sea - which is of immense military importance and through which about $5 trillion worth of shipping trade passes annually - even waters approaching the coasts of the Philippines and other Southeast Asian nations.

China justifies its claims by saying it was the first to have discovered, named and exploited the sea, and outlines its territory using a vague map made up of nine dashes that emerged in the 1940s.

However the tribunal sided with the Philippines in ruling China’s claimed historic rights to resources within the nine-dash map had no legal basis. It also declared that China had acted unlawfully by violating the Philippines’ sovereign rights within its exclusive economic zone - waters extending 200 nautical miles from the Filipino coast.

China had done so by interfering with Philippine fishing and petroleum exploration within the exclusive zone, as well as by building artificial islands there.

China has in recent years undertaken giant land reclamation works in the Spratlys archipelago, one of the biggest island groups in the sea which partly falls within the Philippines’ exclusive economic zone.

The United States and Australia were among Philippine allies this week to quickly call on China to respect the ruling, pointing out it was legally binding.

The Philippines had initially refrained from asking China to abide by the verdict - in line with new President Rodrigo Duterte’s directive to achieve a “soft landing” with Beijing on the issue.

The Philippines filed the legal challenge against China in 2013 under Duterte’s predecessor, Benigno Aquino. Relations between Beijing and Manila plummeted over the row.

Duterte, who took office on June 30, has said he wants better relations with China and to attract Chinese investment for major infrastructure projects.

Unlike Aquino, Duterte has said he wants to talk directly with China over the issue. Beijing also wants to negotiate, but at the same time insists it will never concede on sovereignty.

But the Philippines hardened its stance Thursday with a statement detailing Foreign Secretary Perfecto Yasay’s priorities when he attends a two-day Asia-Europe summit, known as ASEM, in Mongolia along with Chinese Premier Li Keqiang.

“Secretary Yasay will discuss within the context of ASEM’s agenda the Philippines’ peaceful and rules-based approach on the South China Sea and the need for parties to respect the recent decision,” the foreign affairs department said in a statement.

Even just raising the issue at the summit would anger China, which has long bridled at Philippine efforts to have the dispute discussed at multilateral events.

Chinese assistant foreign minister Kong Xuanyou insisted on Monday the ASEM meeting was “not an appropriate venue” to discuss the South China Sea.

But Japanese Prime Minister Shinzo Abe also said Thursday as he left for Mongolia that he wanted the sea to be discussed at the summit.

Vietnam, another claimant in the sea, added to the pressure on Beijing.

“Vietnam asks China to immediately end moves that violate Vietnam’s sovereignty,” Foreign Ministry spokesman Le Hai Binh told reporters in Hanoi.

The Chinese government and state media have this week unleashed a tirade of vitriol against the tribunal, and vowed never to give up claims to the sea. “Do not turn the South China Sea into a cradle of war,” vice foreign minister Liu Zhenmin told reporters in Beijing on Wednesday in a warning to rivals.

He also said China may introduce an air defence zone over the sea, which would give its military authority over foreign aircraft, depending on the “threat” level.

China and the United States, which insists it must help ensure freedom of navigation in the sea, had already deployed significant naval firepower into the disputed waters ahead of the verdict.

Taiwan, which has a very similar claim to the waters as China, sent a warship into the waters on Wednesday to protect its interests.