In opening remarks Tuesday, host President Hun Sen focused on economic issues, urging participants to work to reduce the gap between the region's richest and poorest countries, Voice of America reported.
"Within the region, although the development gap among ASEAN members has been noticeably narrow, it is still huge," Sen said. "This requires [us] to double our efforts to promote further growth and improve equitable distribution of the fruits of growth at both the national and the regional among member countries."
However, long-standing maritime disputes hung over the start of two-day summit, observers said.
The Philippines, Vietnam, Malaysia and Brunei all lay claim to portions of the potentially resource-rich South China Sea, placing them in conflict with China, which claims it all. The Philippines and Vietnam have accused Chinese vessels of breaching their exclusive economic zones and disrupting oil exploration activities.
In a summit pre-meeting, Philippine Foreign Secretary Albert Del Rosario vowed to raise the South China Sea dispute because it "violates the U.N. resolution," the Phnom Penh Post reported.
Del Rosario later urged ASEAN to agree on a code of conduct for the sea before approaching China.
"The Philippines hopes that the code of conduct will be a real move forward not merely in terms of form, but more importantly in substance," he said in a statement.
Del Rosario, however, said the rules-based approach of the U.N. Convention on the Law of the Sea still offered a legitimate path for resolving the issue, the Post said.
ASEAN members also raised concerns about North Korea's planned rocket launch this month, VOA said. North Korean officials have said they are putting a weather satellite into orbit, but the United States and other nations say they think the launch's true purpose is test firing a missile.
Cambodian Foreign Affairs Minister Kao Kim Hourn said Cambodia wants to ensure "peace and stability on the Korean Peninsula," and called for all parties not to "take any action that would escalate tension."
ASEAN comprises Brunei, Cambodia, Indonesia, Laos, Malaysia, Burma, the Philippines, Singapore, Thailand and Vietnam.