The Philippines accused China on Sunday of a "massive" military buildup in the disputed South China Sea, warning at a regional security forum that the Asian giant's tactics were a threat to peace.

The statement by Philippine Foreign Secretary Albert Del Rosario ensured that the growing row over rival claims to the strategically vital and potentially resource-rich sea would again be a key focus of the annual four-day Asia-Pacific talks.

"Del Rosario today expressed serious concern over the increasing militarisation of the South China Sea," said a Philippine government statement released on the first day of the event in the Brunei capital.

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.

Del Rosario described the Chinese presence at these islets as "threats to efforts to maintain maritime peace and stability in the region".

He did not give details of the alleged buildup but 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 declaration on conduct signed by the 10-member Association of Southeast Asian Nations (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, as well as Taiwan, also have competing claims to parts of the sea.

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 of the sea.

Setting the tone for the Brunei event, 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.

Del Rosario on Sunday expressed alarm at such rhetoric.

"The statement on counterstrike is an irresponsible one. We condemn any threats of use of force. We condemn that. And we continue to pursue the resolution of our disputes in a peaceful way," he said.

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 to the sea.

Nevertheless, Indonesian Foreign Minister Marty Natalegawa said ASEAN would continue to press its case with China in Brunei.

"We will be really zeroing in on the need for the code of conduct," Natalegawa told reporters on Saturday.

Toxic smoke from uncontrolled burning of Indonesia's enormous rainforests that has drifted across to neighbouring countries was also discussed on the first day of the Brunei talks.

Natalegawa said on Saturday that the fires had been greatly reduced and were coming under control.

The talks will expand on Monday and Tuesday to include the United States, China, Japan, Russia and other countries across the Asia-Pacific, providing the 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 Russia's Sergei Lavrov and China's Wang Yi.

The United States has been frustrated in recent weeks by perceived Chinese and Russian help for fugitive intelligence leaker Edward Snowden, who is at Moscow's airport after being allowed to leave the Chinese territory of Hong Kong.

Japanese Foreign Minister Fumio Kishida and his South Korean counterpart, Yun Byung-Se, are also set to hold direct talks in Brunei, the first ministerial meeting between the two countries under their new governments.