As the major powers compete for regional influence, it is in ASEAN's interest to remain neutral and united with strong links to all its dialogue partners, Prime Minister Lee Hsien Loong said at Wednesday's summit.
An open and inclusive regional architecture is an important way for ASEAN to enhance its role in promoting peace and security in the region, and one of three things ASEAN must do to continue strengthening itself, he said.
The other two are to press on with building an ASEAN community by 2015, and add organisational muscle by beefing up its secretariat and reviewing processes and structures.
Mr Lee's comments came as US President Barack Obama said the federal government shutdown was hurting the country's credibility abroad, and the Chinese media celebrated President Xi Jinping's star turn at the Apec summit in Bali earlier this week, which the US leader missed.
In his speech to fellow ASEAN leaders, Mr Lee said it was not in ASEAN's interest "to privilege any party or multilateral fora over others in our engagement".
An example of how ASEAN could stay open and inclusive is economic cooperation via avenues like the Regional Comprehensive Economic Partnership (RCEP), a free trade pact it is negotiating with six partners including China and India.
Challenges in the RCEP could be expected given the diversity of interests but ASEAN must maintain its core principles of solidarity, centrality and equality.
On difficult issues like the territorial disputes in the South China Sea, ASEAN takes no stand on the claims' merits as these are up to the claimant states to resolve. But it should continue to call for the early conclusion of a code of conduct (COC), he said, welcoming the start of official consultations by senior officials last month. "We must work on a road map to begin formal negotiations on a COC soon," he added.
Meanwhile, all parties must abide by the 2002 Declaration of Conduct and avoid the use of force - a point he reiterated at the ASEAN-China Summit on Wednesday.
He also said there was scope for China and ASEAN to do more together and welcomed China's proposals to upgrade the ASEAN-China Free Trade Area and set up an Asian Infrastructure Investment Bank. Singapore is the next coordinator of ASEAN-China relations after Thailand.
ASEAN leaders have also tasked its coordinating council to develop a vision for ASEAN post-2015, by the 2015 Summit.
Mr Lee later told reporters that ASEAN must overcome problems and achieve progress "which will make a difference to our people's lives and make us a valuable partner" to other countries. "Then you can be relevant," he said. "It's not what you say, it's what you do."
On Wednesday, Mr Lee also attended the ASEAN summits with Japan, South Korea and US Secretary of State John Kerry. He and South Korean President Park Geun Hye met officially for the first time on the summit sidelines, ahead of his visit to Seoul in December.
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