Myanmar kicked off Wednesday its biggest gathering of world leaders since shedding junta rule but concerns over the pace of democratic reforms are expected to surface at the two-day event featuring US President Barack Obama.

The Association of Southeast Asian Nations (ASEAN) and East Asia summits, held in the remote capital of Naypyidaw, are the culmination of a year of diplomatic limelight for Myanmar after having long been shunted to the sidelines under its former military rulers.

Other key issues will include the festering row between several ASEAN members and China over territory in the South China Sea, and greater economic integration ahead of a Southeast Asian trade union mooted for 2015.

The rise of self-proclaimed Islamic State and countering the Ebola epidemic are also expected to be discussed by the world leaders.

At the end of the ASEAN summit on Wednesday, the leaders will release a statement expressing "concerns over recent developments in the South China Sea, which have increased tensions in the area", according to a draft seen by AFP.

Tensions in the seas have soared this year with Vietnam and the Philippines enraged by series of manoeuvres by regional superpower China in waters claimed by them.

China says most of the seas are its territory -- including areas close to its neighbour's coast.

Beijing is reluctant to sign a binding, multilateral code of conduct covering the disputed and resource-rich waters and experts say there is little prospect of it changing it stance.

In the draft statement, Southeast Asian leaders, including from Muslim-majority Indonesia and Malaysia, also committed to "all necessary measures to prevent fighters travelling from ASEAN member states to join terrorist groups".

ASEAN leaders are due to meet with Chinese Premier Li Keqiang on Thursday during the East Asia Summit, which groups the Southeast Asian bloc with the United States, China, India, Japan, South Korea, Australia, Russia and New Zealand.

-- Obama in Myanmar --

In remarks to open the ASEAN summit, Myanmar's reform-minded President Thein Sein hailed progress on integrating the region's markets and the reduction of trade barriers and tariffs with the goal of an end-2015 economic union fast approaching.

On Wednesday the ASEAN bloc will also hold meetings with India, Japan and the United Nations before Obama lands in the evening fresh from the Asia-Pacific Economic Cooperation (APEC) summit in Beijing.

Obama is set to meet Thein Sein and opposition leader -- and fellow Nobel laureate -- Aung San Suu Kyi during his two-night stay in the country, as he looks to show his support for landmark elections slated for late 2015.

Myanmar has been welcomed back into the international fold after enacting sweeping reforms including the release of most political prisoners and the promise of free and fair polls next year.

The reforms have seen most sanctions lifted, while foreign investment has poured into the almost virgin market of some 60 million people.

Obama two years became the first sitting US president to visit Myanmar, in an initial effort to propel the reforms.

But, in comments just days before Obama's arrival, Suu Kyi sought to temper US "over-optimism" over Myanmar's reform process.

Wrangles over the constitution, the cramping of media freedom as well as tinderbox issues such as burgeoning Buddhist extremism and anti-Muslim violence, have taken the sheen off its emergence from isolation after decades of iron-fisted army rule and raised fears its reforms are slipping.

Myanmar is the second leg of a week-long trip by Obama to push US priorities in the region.

But he arrives bloodied from a major election setback at home and assailed by global crises, from Islamic militants in Iraq and Syria to the conflict in Ukraine and the deadly spread of the Ebola virus.

The United States has said talks on the rise of self-proclaimed Islamic State and Ebola will also be held in Naypyidaw.