Southeast Asian leaders met Tuesday for an annual summit dominated by Myanmar's historic reforms, North Korea's planned rocket launch and strategic maritime disputes with China.

Leaders of the 10-member Association of Southeast Asian Nations (ASEAN) convened in the Cambodian capital two days after by-elections in Myanmar saw pro-democracy leader Aung San Suu Kyi win her first seat in parliament.

Election monitors from Cambodia, which holds the ASEAN chair, have declared Sunday's vote free and fair, and urged the West to lift sanctions imposed over the Myanmar military's long record of rights abuses.

ASEAN foreign ministers applauded the "orderly" conduct of the polls during talks in Phnom Penh on Monday, setting the stage for a strong endorsement from the bloc's leaders at Wednesday's conclusion of the two-day summit.

ASEAN secretary-general Surin Pitsuwan said the vote should contribute to the "reintegration of Myanmar into the global community".

"We are encouraged by the way the by-elections were conducted. We are cheered that all the major stakeholders had cooperated to ensure the success of this landmark political exercise," Surin said in a statement.

"This is only the first step of a long journey."

Myanmar's human rights abuses and iron-fisted suppression of political dissent have often hijacked ASEAN gatherings in the past, much to the embarrassment of more democratic member-states.

But over the past 12 months the country's quasi-civilian government, led by President Thein Sein, an ex-general, has freed hundreds of political prisoners, eased media restrictions and welcomed the opposition back into politics.

At the last ASEAN summit in November, Myanmar was rewarded for its reforms by being promised the bloc's chairmanship in 2014. It is also eager to win greater foreign investment with the prospect of sanctions being lifted.

ASEAN comprises Brunei, Cambodia, Indonesia, Laos, Malaysia, Myanmar, the Philippines, Singapore, Thailand and Vietnam -- a grouping of nearly 600 million people from disparate economic and political systems.

The bloc has often been dismissed as a talking shop but it has assumed new strategic importance in light of Washington's foreign policy "pivot" to Asia and the economic rise of China in recent years.

North Korea's planned rocket launch -- described by Pyongyang as a bid to send a satellite into orbit but condemned by the United States and its allies as a thinly disguised missile test -- is also looming over the Cambodia summit.

Philippine Foreign Secretary Albert del Rosario said the ASEAN foreign ministers spoke with one voice against the nuclear-armed North's launch, expected to take place in mid-April.

The Philippines -- which lies beneath the rocket's expected flight path -- has lodged a formal protest with Pyongyang's representatives at the United Nations.

"I think the countries that spoke on the topic... were all of the opinion that we should be discouraging (North Korea) from undertaking that launch," Del Rosario said Monday.

He said words to that effect would be included in the chairman's statement at the end of the leaders' summit.

Regional tensions with China over disputed islands in the potentially resource-rich South China Sea -- a vital international shipping lane -- also came up during the summit's opening session, diplomats said.

China has competing territorial claims in the sea with ASEAN members Brunei, Malaysia, the Philippines and Vietnam. The United States says it has a "national interest" in keeping the vital trade route open to shipping.

US ally the Philippines has been leading a push for ASEAN to present China with a binding "code of conduct" governing activities in the sea, in a bid to prevent minor incidents from escalating into major conflicts.

But Beijing is insisting it should be involved in the negotiations from the start, diplomats said.

Philippines President Benigno Aquino told his counterparts at the summit that ASEAN should produce a draft code "before involving China", according to Wilfrido Villacorta, the Philippines ambassador to ASEAN.

Cambodian officials have denied that Chinese President Hu Jintao pressured the ASEAN hosts to keep the dispute off the summit agenda during his four-day visit to Phnom Penh on the eve of the regional meeting.

But Prime Minister Hun Sen did not mention the sea in his list of priorities for Cambodia's 2012 chairmanship, which he outlined at the opening of the summit.

"I think it's an open process. It has just begun and it will take some time to (conclude) the drafting of the code of conduct," Surin said.