Bangkok: Chinese diplomatic pressure has forced a formal split in the Association of South-East Asian Nations over how the 10-member group responds to Beijing's claim to almost all of the South China Sea.

The announcement by China's foreign ministry that it has reached a four-point consensus with Brunei, Cambodia and Laos follows a flurry of Chinese moves that appear aimed at pre-empting a ruling expected within weeks by a United Nations-backed tribunal on Beijing's claims in the flashpoint waters expected within weeks.

ASEAN has for years been trying to negotiate a legally binding code of conduct with China to prevent conflict over competing claims by the Philippines, Taiwan, Malaysia, Brunei and Vietnam to the strategic and resource-rich waterways.

The group, which includes Singapore, Thailand and Indonesia, expressed its serious concern in February about growing international tensions over the disputed waters, warning that China's land reclamation and escalating activity by rival claimants could undermine peace.

But China's foreign ministry said Cambodia, Laos and Brunei had agreed that ASEAN states should negotiate disputes with Beijing directly, not through the group, which can only act if all members agree.

The ministry issued a statement declaring the South China Sea problem was not a China-ASEAN dispute and should not affect "China-ASEAN relations".

Cambodia, a close Chinese ally, was accused of driving a wedge into ASEAN in 2012 when its refusal to be drawn on China's actions in the sea resulted in a customary end-of-summit communique not being issued for the first time.

Chinese Foreign Minister Wang Yi last week repeated Beijing's warning that it would reject the findings of the Permanent Court of Arbitration in The Hague on a legal challenge by the Philippines to China's territorial claims.

"Manila's unilateral action is actually a distortion and abuse of the international arbitration mechanism," Mr Wang said.

Carlyle Thayer, an expert on the dispute from the Australian Defence Force Academy in Canberra, said China was likely to mount an international campaign challenging the legitimacy of the tribunal.

He said Australia got a taste of China's likely propaganda offensive when leaders of the Australian-Chinese community issued a statement after a forum early this month warning the Australian political elite to "tread carefully on sensitive matters like the South China Sea" and foreshadowing a possible "crisis situation".

Professor Thayer said recent actions by China suggested the world's most populous nation was preparing to take pre-emptive action in the South China Sea ahead of the ruling, including beefing up air defences on Woody Island.

Professor Thayer pointed to leaked intelligence assessments that China might be planning major construction activities at Scarborough Shoal, which Chinese ships have blockaded since 2012, preventing Filipino fishermen from entering traditional fishing grounds.

He said China's moves were likely to derail any concerted action by the international community to exert pressure on China to respect the tribunal's findings.

"Concerns about international law would be sidelined by China's newest phase of militarisation," Professor Thayer wrote in The Diplomat magazine.

About US$5 trillion ($6.5 trillion) of trade is shipped through the South China Sea each year.

- With agencies