Southeast Asian leaders began annual talks on Sunday that were set to focus on bruising territorial rows, a controversial declaration on human rights and deadly ethnic unrest in Myanmar.
The Association of Southeast Asian Nations (ASEAN) summit was being held following months of acrimony within the 10-member bloc over how to handle disputes with China over rival claims in the South China Sea.
The maritime tensions were expected to be high on the agenda at the summit in Cambodia, as well as during two days of expanded talks starting on Monday that will include US President Barack Obama and Chinese Premier Wen Jiabao.
ASEAN Secretary General Surin Pitsuwan said the bloc was ready to present a united front to China over the issue, after proposing on Saturday a hotline with its giant Asian neighbour aimed at defusing tensions.
"There is a good intention to push the momentum forward," Surin told AFP in an interview on Sunday just ahead of the opening of the talks.
Rival claims to the South China Sea 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.
China insists it has sovereign rights to nearly all of the sea, including waters close to 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.
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.
An ASEAN foreign ministers' meeting in Phnom Penh ended in July without issuing a joint communique for the first time in the bloc's 45-year history because of divisions over how to handle the South China Sea issue.
The Philippines and Vietnam had wanted the communique to make specific reference to their disputes with China but were thwarted by Cambodia, the host of the talks and a close Chinese ally.
In one of the major set pieces for Sunday, ASEAN leaders are scheduled to endorse a declaration they say will enshrine human right protections for the bloc's 600 million people.
However, drafts of the pact have drawn widespread criticism from human rights groups, which say it allows loopholes for governments.
ASEAN's members have a wide range of political systems, from authoritarian regimes in Vietnam and Laos at one end of the spectrum to the freewheeling democracy of the Philippines at the other.
And even as the rights pact is signed, ASEAN leaders will on Sunday have to focus on violence in Myanmar's Rakhine state between Muslim and Buddhist communities that has left 180 people dead since June.
Surin said ASEAN foreign ministers discussed the violence during meetings on Saturday, the same day as the Organisation of Islamic Cooperation branded attacks on the minority Rohingya Muslims as "genocide".
Surin told AFP ASEAN leaders may voice official concern in their end-of-summit statement on Sunday.
Directly before travelling to Phnom Penh, Obama will on Monday make a historic visit to Myanmar to reward and further encourage political developments by the new reformist government there.
Rights groups have also urged him to raise concerns over the violence in Rakhine.
Obama's aides have already said he will raise "grave concerns" in Phnom Penh over Cambodia's rights record and the need for political reform when he meets Prime Minister Hun Sen.
"We'll continue to make clear that we want to see greater political freedom in Cambodia," said Ben Rhodes, a deputy US national security advisor who is travelling with Obama.
ASEAN leaders are also aiming to use the Phnom Penh talks to push forward a planned giant free trade zone with China, Japan, South Korea, India, Australia and New Zealand.
Negotiations for the free trade area, which would account for roughly half the global population and around a third of the world's annual gross domestic product, are set to be officially launched on Tuesday.