The Philippines accused China on Sunday of a "massive" military buildup in the South China Sea and described the Asian giant's tactics as a threat to peace.

The comments by Philippine Foreign Secretary Albert Del Rosario were the latest escalation in a dispute over competing claims to the strategically vital and potentially resource-rich sea that have raised concerns about potential military conflict.

"The Philippines views with serious concern the militarisation of the South China Sea," Del Rosario told fellow foreign ministers from the 10-member Association of Southeast Asian Nations on the first day of a security forum in the Brunei capital.

"The overwhelming presence of Chinese ships, including military and paramilitary ships, and the issuance of threats pose serious challenges for the region as a whole."

In a press statement, Del Rosario said there was a "massive presence of Chinese military and paramilitary ships" at two groups of islets within the Philippines' exclusive economic zone, called Scarborough Shoal and Second Thomas Shoal.

He described the Chinese presence as "threats to efforts to maintain maritime peace and stability in the region".

"We are prevented in our own EEZ (exclusive economic zone) from fishing, from sourcing our own natural resources and from enforcing our own laws," he later told reporters.

Del Rosario said the Chinese actions violated a pact in 2002 in which rival claimants to the sea pledged not to take any actions that may increase tensions.

The non-binding declaration by ASEAN and China also committed claimants to settle their disputes "without resorting to the threat or use of force".

China claims nearly all of the sea, even waters approaching the coasts of neighbouring countries.

ASEAN members the Philippines, Vietnam, Brunei and Malaysia also have competing claims to parts of the sea, as does Taiwan.

The rivalries have for decades been a source of regional tension, with China and Vietnam fighting battles in 1974 and 1988 for control of some islands in which dozens of Vietnamese soldiers died.

Tensions have again grown in recent years with the Philippines, Vietnam and some other countries expressing concern at increasingly assertive Chinese military and diplomatic tactics to stress control over the sea.

A powerful arm of China's state-run media warned the Philippines on Saturday that its defiance could lead to aggressive Chinese action.

"If the Philippines continues to provoke China... a counterstrike will be hard to avoid," said a commentary run by the People's Daily, the mouthpiece of the ruling Communist Party.

ASEAN has been trying for more than a decade to secure agreement from China on a legally binding code of conduct that would govern actions in the South China Sea.

China has resisted agreeing to the code, wary of making any concessions that may weaken its claim.

Chinese Foreign Minister Wang Yi told his ASEAN counterparts that his country had agreed to hold talks on establishing a code during a previously scheduled meeting in Beijing this September.

However Wang cautioned there would be no quick agreement, emphasising that the "consultations" would be a "gradual process".

China's agreement to discuss the issue drew a mixed response from ASEAN. Thai Foreign Minister Surapong Tovichakchaikul described it as a "very significant development".

Indonesian Foreign Minister Marty Natalegawa welcomed the news, but emphasised that diplomacy would only work if all parties first exercised restraint.

"Now there is a sense of anarchy," he said.

The Brunei security talks will expand on Monday and Tuesday to include the United States, China, Japan, Russia and other countries across the Asia-Pacific, providing a platform for face-to-face diplomacy on many of the world's hot-button issues.

US Secretary of State John Kerry is expected to hold a series of rapid-fire meetings with his counterparts from the world's major powers, including Wang and Russia's Sergei Lavrov.