Reclamation work in the disputed South China Sea, where China has constructed an airstrip and other structures on coral reefs, threaten to "undermine peace, security and stability" in the region, Southeast Asian leaders warned Monday.

Beijing's assertion of sovereignty over almost all the sea -- also claimed in parts by several other Asian nations -- has set off alarm bells with its neighbours and beyond as China stakes its claim with growing boldness.

A statement to be issued at the close of the one-day summit in Malaysia notes "serious concerns" over land reclamation on reefs whose sovereignty is contested. The work has triggered fears of tightening Chinese control over the seaway.

"We share the serious concerns expressed by some leaders on the land reclamation being undertaken in the South China Sea, which has eroded trust and confidence and may undermine peace, security and stability," said the statement by summit chair Malaysia, seen by AFP, which did not mention China by name.

Along with Taiwan, Association of Southeast Asian Nations (ASEAN) member-states Vietnam, the Philippines, Malaysia and Brunei claim parts of the sea, which is rich in energy reserves and fishery resources, and is a vital conduit for much of world trade.

Satellite photos released earlier this month provided fresh evidence of the scale of the Chinese programme, depicting a flotilla of vessels dredging sand onto a feature known as Mischief Reef in the Spratly Islands.

Other photos showed a runway and ship harbour taking shape on Fiery Cross, also in the Spratlys, which was little more than a reef when work began late last year.

- China projects its power -

Similar work is taking place at a handful of other sites, according to defence analysts, who say the construction drive will give China a permanent forward presence far out at sea from which to project its growing power.

The closing statement by Malaysia -- which holds the rotating chair of 10-member ASEAN this year -- instructs the region's foreign ministers to "urgently address this matter" under dialogue mechanisms set up between the bloc and China.

But it stopped just short of a call by the Philippine foreign secretary, who on Sunday challenged ASEAN to "finally stand up" to Beijing by demanding an immediate halt to the reclamation.

Albert del Rosario had warned his regional peers in Kuala Lumpur that China was "poised to consolidate de facto control" of the sea.

Malaysia brushed aside suggestions of a stern response that could antagonise China, but its Prime Minister Najib Razak appealed to Beijing to avoid destabilising actions.

"We hope to be able to influence China that it is also to their interest not to be seen as confronting ASEAN and that any attempt to destabilise this region will not benefit China either," he said.

Despite its unity rhetoric, ASEAN members have diverging agendas, and the bloc avoids pushing China too hard on behalf of its members.

Beijing holds immense trade and diplomatic leverage over ASEAN countries, most of which have no stake in the maritime disputes.

The Philippines and Vietnam have experienced the most direct face-offs with China at sea, including a number of tense confrontations in recent years.

Ian Storey, an analyst with the Institute of Southeast Asian Studies in Singapore, said the ASEAN statement was "quite strong" compared to mild past versions.

But he added: "The statement will do absolutely nothing to dissuade China from completing its reclamation programme."

China on Monday said it was willing to work with ASEAN but defended the island-building as being within its "sovereign" territory.

"The accusations made by some countries against China are not reasonable," a foreign ministry spokesman said.

Philippine President Benigno Aquino told fellow regional leaders on Monday that China's actions violate a non-binding 2002 pledge by rival maritime claimants to avoid actions that inflame tensions.

China also is widely believed to be dragging its feet in discussions on turning the earlier pledge into a binding Code of Conduct, so that it can buy time to cement its control at sea.

Aquino said Beijing's actions "pose a threat to the freedom of global commerce and navigation" and cause irreparable harm to the marine environment.