Southeast Asian nations were deadlocked Sunday about how to confront China's territorial claims in the South China Sea as pressure from Beijing again drove a wedge between countries on the region's toughest security challenge.

Their gathering in the Laos capital is the first time regional players -- including China and the United States -- have met en masse since a UN-backed tribunal delivered a hammer blow to Beijing's claim to vast stretches of the sea.

The 10-member Association of Southeast Asian Nations (ASEAN) boasts four countries who have competing claims with Beijing to parts of the sea, and is fiercely divided on the issue.

Rival claimants have accused China of deftly forging alliances with smaller member countries like Laos, this year's host, and Cambodia through aid and loans to divide the once consensus-driven bloc.

Chinese pressure was blamed last month for a startling show of ASEAN discord when countries swiftly disavowed a joint statement released by Malaysia after an ASEAN-China meeting.

That statement had expressed alarm over Beijing's activities in the South China Sea. Cambodia and Laos were later fingered as being behind moves to block it.

Those divisions were on stark display once more in Vientiane on Sunday as regional foreign ministers met for talks.

Insiders accused Cambodia of scuppering moves to include a response to the tribunal ruling in a joint ASEAN communique, which is usually issued at the end of the foreign ministers' meeting.

"We need to put our house in order," one diplomat involved in discussions told AFP Sunday. "But we still have not agreed on anything."

Another ASEAN diplomat added: "We remain deadlocked. We're back to the negotiating table."

Laos described the initial meeting of foreign ministers as "candid and constructive" but by mid-afternoon there was little sign of a breakthrough.

The main sticking point was over whether to refer to the international tribunal ruling and if so how, a Southeast Asia diplomat told AFP.

Some countries are pushing to include a reference that urges all countries to fully "respect diplomatic and legal process", he said -- in line with statements released by the European Union, the US and Japan following the UN-backed decision.

Other countries are opposing any mention of the ruling.

Another diplomat said it was unlikely a statement would be issued on Sunday and if one was published in the coming days it would likely be "really watered down".

- Competing influence -

The impasse in Vientiane has led to fears of a repeat of a 2012 summit in Cambodia where the bloc failed to issue a joint statement for the first time in its history because of disagreements over the South China Sea.

A failure by ASEAN to respond to the tribunal will do little to counter criticism that the bloc risks fading into obscurity as a talking shop with little real diplomatic clout.

The UN tribunal ruling earlier this month infuriated Beijing but was a victory for the Philippines, which brought the case, and for fellow ASEAN members Vietnam, Brunei and Malaysia who also claim parts of the South China Sea.

The bloc's paralysis on how to respond comes as China ratchets up its rhetoric and military manoeuvres in the sea whilst hitting out at the US.

Washington says it takes no position on the territorial disputes but argues for free sea and air passage through what it considers international waters.

US Secretary of State John Kerry arrives in Laos on Monday morning. It is not yet clear whether he will meet his Chinese counterpart Wang Yi, who is also attending.

A State Department official over the weekend said the US would push for participants to ease tensions over the South China Sea and find common ground.

But Chinese Vice Foreign Minister Liu Zhenmin warned ASEAN against being influenced by outside powers, a clear rebuke to Washington.

"They (ASEAN) should in particular guard against the intervention in regional cooperation by big powers outside the region," he said according to the Xinhua news agency.

ASEAN boasts a diverse array of countries ranging from communist one-party states like Laos to the Islamic sultanate of Brunei, military junta-led Thailand and raucous democracies like Indonesia and the Philippines.

Its ability to deal with Beijing is seen as a test of whether it can jointly confront other pressing regional challenges outside of trade.