The Philippines has urged Beijing to respect an international tribunal's ruling that rejected Chinese claims to most of the South China Sea, escalating the row and raising the prospect of conflict.
China reacted furiously to the verdict by a UN-backed tribunal in The Hague, insisting it will ignore the decision while warning its rivals too much pressure on the issue could turn the resource-rich and strategically vital waterway into a "cradle of war".
The Philippines, which launched the legal challenge, had initially refrained from asking China to abide by the ruling - this followed President Rodrigo Duterte's directive to achieve a "soft landing" with the country's much more powerful Asian neighbour.
This is a David-and-Goliath battle, and whatever moral clout the decision gives to the Philippines, it means little for its fishermen trying to make a living near Chinese-occupied reefs.
But Manila hardened its stance in a statement detailing Foreign Secretary Perfecto Yasay's priorities when he attends an Asia-Europe summit, known as ASEM, in Mongolia this week along with Chinese Premier Li Keqiang.
"Secretary Yasay will discuss ... the Philippines' peaceful and rules-based approach on the South China Sea," the foreign affairs department said in a statement.
The statement said Yasay would also discuss "the need for parties to respect the recent decision".
Even just raising the issue at the two-day summit starting on Friday will anger China, which has long bridled at Philippine efforts to have the dispute discussed at multilateral events.
Chinese Assistant Foreign Minister Kong Xuanyou insisted the ASEM summit was "not an appropriate venue" to discuss the South China Sea.
But China appears to be in the minority - Japanese Prime Minister Shinzo Abe has also said he wanted to discuss the South China Sea at the summit.
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, The sea is also rich in natural resources, including oil.
China even claims waters approaching the coasts of the Philippines and other South East 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 surfaced in the 1940s.
However the tribunal in The Hague ruled 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, the court ruled.
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 30 June, has said he wants better relations with China and to attract Chinese investment for major infrastructure projects.
Unlike the former president, 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.