Philippine Defence Secretary Delfin Lorenzana yesterday slammed what he called Beijing's doublespeak, after its top envoy in Manila said China would "not take the first shot" in the South China Sea, even as it expands its military presence in disputed waters.

"So they are saying… they want peace in the South China Sea, blah blah blah. But it does not match what they are doing on the ground," Mr Lorenzana told reporters.

He said he is sceptical each time China claims it does not bully other nations that are claiming parts of the South China Sea.

"It's good (to hear)... Maybe it's meant to keep us calm… But the bottom line is what they're saying does not match what they're doing," he said.

Mr Lorenzana's remarks come as top diplomats, including United States Secretary of State Mike Pompeo, converge in Bangkok this week for a key South-east Asia summit to tackle rising tensions in the South China Sea and the fallout from the US-China trade war.

"The South China Sea will be an important agenda item. They (China) will be seeking to curb any further hardening of stance by the Philippines," Mr Alexander Neill, an expert on Chinese military affairs at the International Institute for Strategic Studies, told Bloomberg.

"They will likely reiterate their sovereignty over the islands and reefs and criticise external interference," he said.

Mr Lorenzana said China's occupation of the Scarborough shoal, just 358km west of the Philippines' main coast, was proof Beijing's intentions had not been benevolent.

"That was bullying," he said.

The US has also described China's more aggressive posture in asserting its claim to vast swathes of the South China Sea as "bullying behaviour".

Mr Lorenzana was responding to China's Ambassador to the Philippines Zhao Jianhua, who said in a speech on Monday to mark the 92nd anniversary of the People's Liberation Army that Beijing "adopts a military strategy of active defence, which adheres to the principle of defence, self-defence and post-strike response".

"We will not take the first shot," Mr Zhao said.

China has been in control of Scarborough since 2012, after a Philippine Navy frigate intercepted eight Chinese fishing boats suspected of poaching coral and giant clams.

A two-month stand-off between the Philippines and China later ensued.

The US eventually mediated a deal. Both sides were told to withdraw from Scarborough.

The Philippines pulled out its ships. But China stayed, and later sealed off the entire atoll.

Last week, Mr Lorenzana complained that Chinese warships had been making unannounced passage through the Philippines' southernmost island en route to the Pacific since February.

"They should inform us that they are passing," he told reporters.

China claims nearly all of the South China Sea, where about US$5 trillion (S$6.85 trillion) sea-borne trade passes annually.

Brunei, Malaysia, the Philippines, Vietnam and Taiwan also have claims on parts of the sea.

China has turned seven reefs it occupied in the southern half of the South China Sea into islands to secure its claims.

Satellite and aerial surveillance has found emplacements for missiles, 3km-long runways, extensive storage facilities, and a range of installations that can track satellites as well as foreign military activity and communications on these islands.

The envoy, Mr Zhao, said in his speech that China "is the last country" that would want freedom of navigation and overflight in the South China Sea curtailed.

Three-fourths of China's trade passes through these waters, including 80 per cent of its oil needs, he said.