Foreign ministers from the 10-member Association of Southeast Asian Nations are gathering in the Cambodian capital before being joined later this week by US Secretary of State Hillary Clinton and 16 other counterparts including China, Japan and the Koreas for the ASEAN Regional Forum.
Tension over competing claims in the South China Sea promises to be the hot button issue, with Indonesia's foreign minister saying he expected to see movement on efforts to draft guidelines to resolve disputes in the sea.
"If not now, when? I mean, this is what we are here for," Marty Natalegawa told reporters in Phnom Penh on Sunday.
Manila is leading a push for ASEAN to unite to persuade China to accept a "code of conduct" (COC) in the sea, where tensions have flared recently with both Vietnam and the Philippines accusing Beijing of aggressive behaviour.
China has preferred an approach that would deal with the claimants individually.
"We will be discussing where we are and how to proceed, as to when for example to engage China in the process of formulating this code of conduct," Natalegawa said, adding that "recent developments" in the sea would also be on the agenda.
China, Taiwan and ASEAN members the Philippines, Vietnam, Brunei and Malaysia have overlapping territorial claims in the South China Sea, home to vital shipping lanes and believed to be rich in oil and gas deposits.
China recently angered Vietnam by inviting bids for exploration of oil blocks in contested waters, sparking protests in Hanoi, while Beijing and Manila are locked in a tense standoff over a disputed shoal.
Clinton on Sunday urged "progress" on a code of conduct in the South China Sea.
Strategic rivalry between Washington and Beijing is expected to loom large over the summit, following the recent expansion of US military relations with the Philippines and Vietnam.
But observers predict Clinton will be eager to downplay any friction with China and may be less outspoken on the maritime dispute than in the past.
ASEAN comprises Brunei, Cambodia, Indonesia, Laos, Malaysia, Myanmar, the Philippines, Singapore, Thailand and Vietnam -- a grouping of nearly 600 million people from disparate economic and political systems.
The bloc has often been dismissed as a talking shop but it has assumed new strategic importance in light of Washington's foreign policy "pivot" to Asia and the economic rise of China in recent years.