Southeast Asian leaders endorsed a controversial human rights pact on Sunday at an annual summit in which they also sought to step up pressure on China over a bruising territorial dispute.

Heads of the 10-member Association of Southeast Asian Nations (ASEAN) hailed their declaration on human rights as a landmark agreement that would help protect the region's 600 million people.

"It's a legacy for our children," Philippine Foreign Secretary Albert del Rosario told reporters after the signing ceremony.

But critics said it allowed too many loopholes for ASEAN, which groups a diverse range of political systems ranging from authoritarian regimes in Laos and Vietnam to freewheeling democracies such as the Philippines.

"Our worst fears in this process have now come to pass," said Human Rights Watch deputy Asia director Phil Robertson.

On the day the pact was signed, leaders were having to discuss the ethnic violence in ASEAN member Myanmar, where clashes in Rakhine state between Muslim and Buddhists have left 180 people dead since June.

ASEAN secretary-general Surin Pitsuwan told AFP on Sunday that the violence was disturbing and risked destabilising the region.

He said leaders would discuss the bloodshed and potentially include a statement referring to it in their end-of-summit communique.

The ASEAN event will be expanded into a two-day East Asia Summit starting Monday that includes the leaders of the United States, China, Japan, India, South Korea, Australia, New Zealand and Russia.

US President Barack Obama is due to arrive in Phnom Penh on Monday after making an historic visit to Myanmar.

Obama decided to make the trip to Myanmar, the first by a sitting US president, to reward and further encourage political developments by the new reformist government there.

However the Organisation of Islamic Cooperation, which on Saturday described the Muslim Rohingya minority as victims of "genocide", has urged Obama to pressure Myanmar's government to stop the bloodshed.

During their summit on Sunday, ASEAN leaders forged a united position on negotiating with China over competing territorial claims to the strategically vital South China Sea.

China insists it has sovereign rights to nearly all of the sea, including waters near the coasts of its Asian neighbours.

ASEAN members Vietnam, the Philippines, Malaysia and Brunei, as well as Taiwan, also have sometimes overlapping claims to the sea.

The rival claims have for decades made the waterways, home to some of the world's most important shipping lanes and believed to sit atop vast natural resources, a potential military flashpoint.

Tensions escalated this year amid complaints by the Philippines and Vietnam that China was becoming increasingly aggressive in staking its claim to the sea, including by employing bullying diplomatic tactics.

After infighting among ASEAN members on how to deal with the issue, Surin said the bloc would propose to Chinese Premier Wen Jiabao at the East Asia Summit that they quickly begin top-level talks on the issue.

He said ASEAN leaders had agreed on the main negotiating points for China in regards to a long-awaited code of conduct aimed at governing behaviour and reducing tensions in the South China Sea.

"On the ASEAN side, (we are) ready, willing and very much committed but it takes two to tango," Surin told reporters.

But China offered no signs of a change in position as foreign ministry spokesman Qin Gang restated Beijing's preference to deal directly with rival claimants rather than ASEAN as a bloc.

"We are continuing to discuss with ASEAN countries and particularly with those countries related (rival claimants)," he told reporters in Phnom Penh.