President Barack Obama's attendance at an annual summit of Southeast Asian leaders on Tuesday thrust him right in the eye of the region's most stormy dispute: the long-raging rivalry between China and five neighbors for control of strategic and resource-rich waters in the South China Sea.

The inability to resolve these territorial conflicts has become a major impediment to the 10-member Association of Southeast Asian Nations as it tackles ambitious dreams like a plan to turn the economically vibrant region of 600 million people into an EU-like community by the end of 2015.

Neither the U.S. nor China is a member of ASEAN, but each has strong supporters in the group. Summit host Cambodia, an ally of China, has tried at this week's summit to shift the focus to economic concerns, but Beijing's territorial disputes with its ASEAN neighbors _ including staunch U.S. ally the Philippines _ have yet again overshadowed discussions.

The disagreement sparked a tense moment Monday when Philippine President Benigno Aquino III challenged Cambodian Prime Minister Hun Sen, who had tried to cut off a discussion of the territorial disputes.

Into this heated atmosphere came Obama, who flew to Phnom Penh for Tuesday's expanded East Asia Summit, in which the 10 ASEAN countries were joined by eight other nations, including China and the United States. Behind closed doors, the Chinese and Philippine leaders pressed their territorial claims while others called for restraint. After the summit, the exchange shifted to the chandelier-lit lobby, where diplomats of the two countries reiterated their positions.

Senior Chinese diplomat Fu Ying expressed dismay that the disputes again got the spotlight. "We do not want to bring the disputes to an occasion like this and we do not want to give over-emphasis to the territorial disputes and differences," she said.

Washington has reiterated that it takes no sides in the territorial disputes but would not allow any country to resort to force and block access to the South China Sea, a vital commercial and military gateway to Asia's heartland.

Washington has also called for the early crafting of a "code of conduct" to prevent clashes in the disputed territories, a call backed by Australia and Japan, but it remains unclear if and when China will sit down with rival claimants to draft such a legally binding nonaggression pact.

The potentially oil- and gas-rich South China Sea islands and waters are contested by China, Taiwan and four ASEAN members _ Brunei, Malaysia, the Philippines and Vietnam.

The last fighting, involving China and Vietnam, killed more than 70 Vietnamese sailors in 1988, and fears that the conflicts could spark Asia's next war have kept governments on edge.

Vietnam and the Philippines, backed by Washington, have been loudest on the issue, and want China to negotiate with the other claimants as a group. China wants one-on-one negotiations _ which would give it an advantage because of its sheer size and economic clout _ and has warned Washington to stay away from an issue it says should not be "internationalized," a position echoed by Cambodia at the Phnom Penh summit.

There have been several recent standoffs involving boats and other shows of force, particularly between China and the Philippines, which both claim ownership of the Spratly Islands, a spray of tiny South China Sea atolls.

Their latest diplomatic confrontation occurred a few hours before Obama touched down Monday in the Cambodian capital, when Hun Sen announced as he was closing a meeting that all ASEAN leaders had agreed not to discuss the divisive issue in talks between the 10-nation bloc and China.

Alarmed, Aquino raised his hand, stood up and objected to Hun Sen's statement, saying his country, which plans to bring the disputes before a U.N. tribunal, was not party to any such agreement. It was a blunt gesture in the usually servile ambiance of the conservative bloc, an unwieldy collective of rigid, authoritarian regimes and nascent democracies.

After a brief lull, Hun Sen recovered and said Aquino's remarks would be reflected in the record of the meeting. Still, Cambodian and Chinese officials insisted that the agreement stood. Tensions intensified Tuesday when the Philippines was joined by Vietnam and Singapore in objecting to a plan by Cambodia to state in a post-summit statement by the host country that there was indeed such an agreement, Philippine diplomats said.

An objection from the Philippines, or any ASEAN nation, ought to be enough to thrash any agreement because the bloc decides by consensus, meaning just one veto from any member kills any proposal.

"The bottom line is they can talk all they want but if we said we're not with it, there's no consensus, finished," Philippine Foreign Secretary Albert del Rosario told reporters.

The dispute, and Obama's presence here, highlights how ASEAN has become a major battleground for influence in Asia, just like the South China Sea. The U.S. is pushing its "Pacific pivot" to the region following years of engagement in Iraq and Afghanistan. China, the Asian superpower, has acted to protect its home ground.

Southeast Asia is clearly pinned in between, and the lack of consensus among the group over the maritime disputes has pushed much of the bloc's other work to the sidelines.

In July, after a foreign ministers' meeting also hosted by Cambodia, the group failed to publicly issue a traditional after-conference communique _ an embarrassing failure that was a first in ASEAN's 45-year history. Vietnam and the Philippines had insisted that the joint statement simply state that the South China Sea rifts were discussed, but Cambodia adamantly refused, echoing China's line to keep a lid on public discussions of the disputes.

Ernest Bower of the Center for Strategic & International Studies in Washington, D.C., said the imbroglio in July showed that as long as any ASEAN country remains weak and vulnerable to muscling from a major power, the entire group could be compromised.

"ASEAN learned a hard lesson from the event," Bower said, "namely, that they should never again allow a fellow ASEAN member country to feel so isolated, exposed or dependent on any foreign power that the country feels compelled to step beyond ASEAN protocols ... in a way that damages the organization's interests and profile."

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Associated Press writers Grant Peck and Sopheng Cheang contributed to this report.