Diplomats say China was put on the defensive Friday when numerous delegates at the ASEAN Regional Forum brought up a territorial dispute over the Spratly islands.
One delegate told Reuters news agency that Chinese Foreign Minister Yang Jiechi was "clearly exasperated" by the onslaught behind closed doors.
Diplomats said 12 of the 27 participants called for a multilateral approach to maritime issues such as the Spratlys, all or part of which are claimed by Forum members China, Brunei, Malaysia, the Philippines and Vietnam.
China has increased its naval presence in the region, which is believed to hold potentially rich oil and gas deposits. It prefers to deal with each country on an individual basis.
U.S. Secretary of State Hillary Clinton was among those to raise the issue.
In prepared remarks, she said the United States has "a national interest in freedom of navigation, open access to Asia's maritime commons, and respect for international law in the South China Sea."
Some information for this report was provided by AP, AFP and Reuters.