Southeast Asian leaders have expressed "serious concern" over worsening territorial disputes in the South China Sea, presenting a rare united front against an increasingly assertive Beijing.

Vietnam and the Philippines led a successful push for the Association of Southeast Asian Nations to deliver a thinly veiled rebuke to China over the standoff in waters home to key shipping lanes and thought to contain huge energy reserves.

But a defiant Beijing said Hanoi's efforts to enlist the support of its neighbours in the row were "doomed to fail".

The 10-nation ASEAN, in a statement released on Monday after a summit on Sunday, called for a peaceful resolution to the maritime rows, which flared up this month after China moved an oil drilling rig into waters also claimed by Hanoi.

"We expressed serious concerns over the ongoing developments in the South China Sea," said the joint statement from the summit in Myanmar, without explicitly pointing the finger at Beijing.

ASEAN called on all parties involved to "exercise self-restraint, not to resort to threat(s) or use of force, and to resolve disputes by peaceful means in accordance with the universally recognised principles of international law".

Observers said the statement marked a change of tone by the regional bloc, many of whose members -- including Myanmar -- have close economic and political ties with China and have traditionally avoided confrontation with the Asian heavyweight.

In 2012 China's ally Cambodia caused consternation when it was ASEAN head by refusing to take Beijing to task over its assertive maritime stance.

"This is a far cry from when Cambodia was ASEAN chair," said Southeast Asia expert Carl Thayer, a professor at the University of New South Wales in Australia.

The statement "represents a slight tightening of ASEAN's position", he said, adding it suggests a rare level of "consensus" on the vexed sea rights issue.

Under Brunei's chairmanship last year, China avoided a public rebuke from ASEAN at a major summit after offering an olive branch by calling for peace in the flashpoint region.

Beijing struck a less conciliatory tone on Monday, insisting that the contested Paracel Islands, located near the controversial oil rig, were its "inherent territory".

"The facts show that Vietnam's efforts to rope others into pressuring China are doomed to fail," foreign ministry spokeswoman Hua Chunying told reporters in Beijing.

"We hope that the Vietnamese side will see the situation clearly, calmly face the facts and stop its disruptions of Chinese operations," she said.

- Balancing act -

Prominent Vietnamese political commentator Nguyen Quang A described the ASEAN declaration as a "big positive" for his country.

Vietnam lobbied energetically at the latest meeting in Myanmar for a strong statement on the maritime issue from its neighbours.

Vietnam's Prime Minister Nguyen Tan Dung slammed Beijing's oil rig move as "extremely dangerous" and accused Chinese vessels of ramming Vietnamese ships in the disputed waters.

"This is the first time China brazenly brings and installs its drilling rig deep into the continental shelf and exclusive economic zone of an ASEAN country, which gravely violates the international law," he said, according to an official transcript of his speech.

The spat triggered large anti-China protests in Vietnam at the weekend that appeared to have the tacit blessing of the authoritarian communist regime in Hanoi, which usually limits expression of public discontent.

The demonstrations received unprecedented coverage in Vietnam's tightly controlled state media on Monday.

Analyst Quang A said more rallies were likely unless Beijing removes the drilling rig.

"Our fate is that we are situated here, with a big neighbour who always wants expansion," he said.

China and Vietnam fought a brief border war in 1979 and the pair frequently trade diplomatic barbs over oil exploration, fishing rights and the contested Spratly and Paracel Islands.

China asserts ownership over almost all of the South China Sea, which is also claimed in part by ASEAN members Vietnam, the Philippines, Brunei and Malaysia as well as Taiwan.

Philippine President Benigno Aquino told reporters that "many leaders" at the summit had voiced concern about the South China Sea spats, which he said were a "cause for worry and concern by all parties".

Philippine prosecutors said Monday they would charge nine Chinese fishermen arrested in separate disputed waters with environmental crimes, despite Beijing's warning of a dire effect on relations.