BEIJING China told the United States not to interfere in a regional dispute over claims to the South China Sea, saying it would only complicate the matter.
Japan's NHK TV reported last week that the United States and southeast Asian countries may announce a joint statement on September 24 that obliquely presses China over its recent activities near disputed isles in the South China Sea.
China has been increasingly strident in asserting its territorial claims, especially maritime ones.
In the past week, it has suspended high-level exchanges with Japan and promised tough counter-measures after a court there extended the detention of the captain of a Chinese boat which collided with two Japanese coastguard ships near disputed islands.
"We express great concern about any possible South China Sea announcement made by the United States and the ASEAN countries," Foreign Ministry spokeswoman Jiang Yu told a regular news briefing. ASEAN is the 10-member Association of Southeast Asian Nations.
"We resolutely oppose any country which has no connection to the South China Sea getting involved in the dispute, and we oppose the internationalization, multilateralization or expansion of the issue. It cannot solve the problem, but make it more complicated," she said.
Washington has criticized Chinese claims to swathes of the South China Sea, where Taiwan and several ASEAN members including Vietnam, Malaysia and the Philippines also assert sovereignty.
China says it has sovereignty over the seas, home to valuable fishing grounds and largely unexploited oil and natural gas fields.
It reacted with anger in July when the United States brought up the issue at a regional meeting, further souring ties between Beijing and Washington already under strain from spats over the value of the Chinese currency, Tibet and Taiwan.
While there have been no military clashes in the seas for years, China and some of the other claimants have been building up their military presence in the region.
(Reporting by Huang Yan and Chris Buckley; Writing by Ben Blanchard, editing by Jonathan Thatcher)