Hosting 19 leaders of the ASEAN countries for the summit, Obama said, "Few regions present more opportunity to the 21st century than the Asia Pacific."

"Together, we can also continue to increase our security cooperation to meet shared challenges," he said as he welcomed leaders of Thailand, Indonesia, Malaysia, the Philippines, Singapore, Brunei, Vietnam, Laos, Myanmar and Cambodia at Rancho Mirage in California.

In recent years, the US has increased its maritime security assistance to its allies and partners in the region, improving mutual capabilities to protect lawful commerce and to respond to humanitarian crisis, he said.

This is for the first time that the US President has hosted ASEAN leaders for a summit meeting.

Chinese assertiveness in the South China Sea, where China and several Southeast Asian states have conflicting and overlapping claims, is expected to figure in the joint declaration of two-day summit.

"I'm very confident that, among other topics that we will discuss during the next day and a half, this (the South China Sea) will be an important one -- by no means the only one," National Security Advisor Susan Rice told reporters.

"We will be continuing to work with our ASEAN partners on a potential statement that we might issue together. This statement will cover a wide range of topics. It won't be focused primarily on the South China Sea," she said in response to a question.

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