PHNOM PENH, Cambodia — Disputes in the strategically important South China Sea proved so contentious here that an annual regional gathering has ended without even a basic diplomatic communiqué, which appeared to have been blocked by China.

The host for the conference of the Association of Southeast Asian Nations, Cambodia, a close ally of China, refused to play the customary role of seeking agreement among the 10 participating countries, thus undermining the possibility of an accord, a senior diplomat from the association said Thursday.

“China bought the chair, simple as that,” said the diplomat, who declined to be identified publicly according to usual protocol. The diplomat pointed to an article on Thursday by China’s state news agency, Xinhua, in which the country’s foreign minister, Yang Jiechi, was quoted as thanking Cambodia’s prime minister for supporting China’s “core interests.”

Secretary of State Hillary Rodham Clinton, who met with foreign ministers at the conference of the Southeast Asian group, said in remarks clearly aimed at China that it was important that the disputes be resolved “without coercion, without intimidation, without threats and without use of force.”