(Bloomberg) -- Vietnam and the Philippines raised concerns over repeated violations of maritime rules at a summit of Southeast Asian leaders amid growing grievances with China over its territorial claims in the South China Sea.

The meeting, held virtually and hosted by Vietnam, comes as the region reopens after lockdowns to check the spread of Covid-19 and negotiates travel lanes with other nations. While the 10-nation bloc known as the Association of Southeast Asian Nations has pledged cooperation to fight the virus, it is also grappling with Beijing’s assertions in the high seas.

“While the entire world is stretched thin in the fight against the pandemic, irresponsible acts and acts in violation of international law are still taking place, affecting the environment of security and stability in certain regions, including our region,” Vietnam’s Prime Minister Nguyen Xuan Phuc said in his opening remarks in Hanoi without mentioning China directly.

Phuc said that while the region promotes the “full and strict compliance” of the rules governing the South China Sea, and is “making every effort to establish an effective” code of conduct with China, problems still remain. “International institutions and international law are being seriously challenged,” he said.

He was not the only regional leader to air concerns about Beijing’s moves in the South China Sea. Philippines President Rodrigo Duterte called on all involved “parties” to follow the laws that govern the sea, particularly the United Nations Convention for the Law of the Sea, known as Unclos.

“Even as our region struggled to contain Covid-19, alarming incidents in the South China Sea occurred,” he said. “We call on the parties to refrain from escalating tension, and abide by responsibilities under international law.”

Stretching from China in the north to Indonesia in the south, the waterway encompasses 1.4 million square miles (3.6 million square kilometers). China claims more than 80% of the South China Sea and backs up its claim with a 1947 map that shows vague dashes -- the so-called nine-dash line. Vietnam, the Philippines, Brunei, Malaysia and Taiwan claim parts of the same maritime area.

In recent years, the Philippines and Vietnam have become the most vocal opponents to China’s claims in the areas that overlap with their own. But despite several run-ins with Chinese ships, they have to weigh their response against the prospect of losing the backing of one of the region’s biggest investors. Over the weekend, for example, the Philippines’ Department of Energy said it is pushing to proceed with a joint exploration with China for resources in the disputed sea.

U.S.-China Tensions

But Southeast Asian leaders must also account for another major power on whom they depend -- the U.S.

In recent months, the U.S. has said it will prioritize the deployment of its forces to the Asia-Pacific region from other areas in the face of growing competition with China. That is raising the chances of a conflict in the South China Sea, experts have said.

U.S.-China Confrontation Risk Is Highest in the South China Sea

“We’re going to make sure we’re postured appropriately to counter” the People’s Liberation Army, U.S. Secretary of State Michael Pompeo said in remarks on Thursday. “We think that’s the challenge of our time, and we’re going to make sure we have resources in place to do that.”

As Washington and Beijing continue to exchange barbs over everything from trade and Covid-19 to supply lines and Hong Kong, several Asean leaders on Friday called for the need for the bloc to work closely together.

Asean should cooperate to “strengthen regionalism, avoid being forced to take sides and become a bridge for those powerful nations to interact with the region,” Thai Prime Minister Prayuth Chan-ocha said. “Use Asean’s view on Indo-China to the maximum benefit.”

©2020 Bloomberg L.P.