US Secretary of State Hillary Clinton has joined the Asean regional summit, amid a continuing war of words between China and Japan over disputed islands.
An apparently friendly China-Japan meeting ahead of the Hanoi summit on Friday was followed by Beijing accusing Tokyo of damaging bilateral relations.
The US has urged dialogue but China has already expressed dissatisfaction with Mrs Clinton over the issue.
Mrs Clinton's agenda will also include human rights issues.
The 16-nation East Asia Summit has now formally opened in the Vietnamese capital.
The China-Japan row is over islands known in Japan as Senkaku and in China as Diaoyu. They are controlled by Japan but claimed by China.
Ahead of her arrival, Washington called on the nations to "have a thoughtful, considered dialogue and resolve these issues".
State department spokesman PJ Crowley said: "We recognise that there is an open question of sovereignty, and we expect that to be resolved between Japan and China through dialogue."
But China has already said it is "strongly dissatisfied" with Mrs Clinton for speaking out on the dispute after meeting her Japanese counterpart Seiji Maehara in Hawaii on Thursday.
She said the islands fell within the scope of Japan's security alliance with the US.
Chinese Assistant Foreign Minister Hu Zhengyue said on Friday: "The Japanese side should take responsibility for ruining the atmosphere for leaders of the two countries."
China faces opposition from a number of Asean members over its claims in the South China Sea and the matter may arise again on Saturday.
Mr Crowley said Mrs Clinton would raise human rights concerns with the hosts.
"There have been some recent instances where journalists, bloggers, other activists have been arrested. This is contrary to Vietnam's own commitment to internationally accepted standards of human rights, including the freedom of speech," Mr Crowley said.
He also said the issue of detained Chinese Nobel Peace Prize winner Liu Xiaobo would come up in discussions with China.