The three-day meeting of the 10-member ASEAN starts in the Laotian capital Vientiane on Thursday.

US Secretary of State Antony Blinken will meet his Chinese counterpart Wang Yi on the sidelines of the event where he will "discuss the importance of adherence to international law in the South China Sea", according to the US State Department.

Beijing claims the waterway, through which trillions of dollars of trade passes annually, almost in its entirety despite an international court ruling that its assertion has no legal basis.

A series of clashes between Philippine and Chinese vessels at flashpoint reefs in recent months have fueled fears of a conflict that could drag in the US owing to its mutual defense treaty with Manila.

ASEAN ministers are expected to issue a joint communique after their meeting on Thursday.

In a draft seen by AFP, some ministers expressed concerns over "serious incidents" in the waterway "which have eroded trust and confidence, increased tensions and may undermine peace, security and stability in the region".

A Filipino sailor lost a thumb in a June 17 confrontation when Chinese coast guard members wielding knives, sticks and an axe foiled a Philippine Navy attempt to resupply its troops on a remote outpost.

Beijing and Manila later reached an agreement allowing for the resupply of the troops stationed on a rusty warship deliberately grounded on the Second Thomas Shoal in 1999 to assert Manila's claims to the area.

One diplomat who is attending the meeting in Vientiane said China's assertiveness in the sea was pushing some Southeast Asian countries closer to the US. 

Diplomats in the region were also preparing for the possibility of a Donald Trump victory in November's US election, they said, requesting anonymity to speak to the media. 

ASEAN countries "more or less have a feel of how to deal with him. [...] They know what his trigger points are, what he likes, what he dislikes," he said.

Read also: Why is the South China Sea a flashpoint for Asia?

Also on the agenda in Vientiane is the civil war in Myanmar, sparked by a military coup in 2021.

ASEAN, of which Myanmar is a member, has led diplomatic efforts to resolve the crisis but has made little progress.

The junta is excluded from the bloc's top-level meetings over its refusal to negotiate with its opponents and its brutal crackdown on dissent. 

Myanmar is expected to send a senior bureaucrat to this week's meeting, according to several sources.

The military's readiness to reengage with ASEAN diplomatically was a "sign of the junta's weakened position", a Southeast Asian diplomat, who will attend the talks, told AFP on condition of anonymity.

The generals have yet to make any meaningful counterattack following an offensive by ethnic armed groups in October that seized swaths of territory along the border with China. 

The losses triggered rare public criticism of its top leadership.

"The center is still solid under the junta," the diplomat said, warning Myanmar could "become a failed state".

The draft communique seen by AFP said ministers "strongly condemned" the continued violence.

The crisis has divided the bloc, with Indonesia, Malaysia and the Philippines calling for tougher action against the junta.

Thailand has held its own bilateral talks with the generals as well as detained democracy leader Aung San Suu Kyi.