The 16-nation East Asia Summit opened in Vietnam's capital Saturday, with US Secretary of State Hillary Clinton and Russian counterpart Sergei Lavrov also joining the talks.
The US and Russia will be formally invited as members of the grouping at its annual gathering, in what analysts say is a blow to Chinese attempts to diminish US influence in the region.
Their entry into the EAS, which elevates its diplomatic heft, comes despite Chinese attempts to promote another grouping -- which does not include the US -- as the region's premier forum for regional cooperation.
US membership is seen as part of its strategic return to Southeast Asia to balance China's growing influence in the region, where Beijing's more aggressive stance on territorial disputes has unnerved its smaller neighbours.
Clinton, in a speech on Asia-Pacific relations made in Honolulu earlier this week, downplayed suggestions the US is duelling with China for influence.
"There are some in both countries who believe that China's interests and ours are fundamentally at odds. They apply a zero-sum calculation to our relationship. So whenever one of us succeeds, the other must fail," she said.
"But that is not our view."
The East Asia Summit is a forum for dialogue on strategic, political and economic issues involving the 10-member Southeast Asian bloc as well as Australia, China, India, Japan, South Korea and New Zealand.