US Secretary of State Hillary Clinton and Chinese Foreign Minister Yang Jiechi are due to meet at the Asean forum, amid regional tension.
A fresh dispute erupted this week between China and Japan over a string of islands in the East China Sea.
Meanwhile, China has reacted with caution to an Asean code of conduct on disputes in the South China Sea.
Clinton is on a tour of Asia which analysts say signals Washington's growing interest in the region.
She had earlier said that the US was making ''a pivot toward Asia".
Clinton arrived in the Cambodian capital, Phnom Penh, from Laos, where she made history as the first top US diplomat to visit the country in nearly six decades.