MANILA: China has opposed a Philippines-led push for a review of its 70-year-old defence treaty with the United States, Manila's defence minister said on Thursday, concerned that it could be seen in Beijing as an effort to contain its rise.

The Philippines is keen to amend the 1951 Mutual Defense Treaty (MDT) to make clear the extent to which the United States would protect and defend its ally should it come under attack.

At an event to mark the MDT's 70th anniversary, Defence Secretary Delfin Lorenzana said he had been urged by a former Chinese diplomat to back off.

"While the U.S. welcomes the idea of revisiting the MDT, an outside party does not," he said.

"The former Chinese ambassador came to me and said: 'Please do not touch the MDT. Leave it as it is,'" said Lorenzana.

He later clarified the conversation took place in 2018.

"It did surprise me. I asked him why? He said any attempt to revise the MDT would be construed by the Chinese government as act to contain the rise of China," Lorenzana told Reuters.

Asked how he responded, Lorenzana said: "I just looked at him and smiled".

There was no immediate comment from the Chinese embassy in Manila.

The push for clarity on Washington's commitment comes amid a rapid buildup of Chinese maritime assets in contested areas of the South China Sea, including what the Philippines says is a militia disguised as a massive fishing fleet near Beijing's militarised manmade islands.

The Philippines has filed dozens of diplomatic protests about the militia and announced it would send another on Thursday.

The Philippine-U.S. alliance has existed for decades, with a rotating presence of U.S. troops for joint exercises, intelligence exchanges and hardware transfers.

Lorenzana said it was clear that strengthening the MDT was not in China's interests.

"The Chinese, having embedded themselves with their artificial islands, are not in a hurry for any resolution," he told the forum.

"It knew that any aggression it takes will trigger the MDT."