Philippine fishermen on Wednesday criticised President Rodrigo Duterte's decision to ban them from a rich South China Sea fishing ground, part of his efforts to ease tensions over disputed waters controlled by China.

Duterte's aides said he had made a "unilateral" declaration to make the lagoon at Scarborough Shoal a protected marine sanctuary, after raising the issue during a meeting with Chinese President Xi Jinping on the sidelines of a regional summit in Peru last weekend.

It was not clear whether Xi supported the plan.

The two neighbours both claim the shoal as part of their territory. But China took control of the ring of reefs just 230 kilometres (140 miles) from the main Philippine island of Luzon in 2012 after a standoff with the Philippine navy.

"We fear that declaring it as a marine sanctuary would pave way for another fishing blockade," Fernando Hicap, chairman of fishermen support group Pamalakaya, said in a statement.

"This time it will be our own law and government that will prohibit (Filipino fishermen), not China," he added.

After taking control of the shoal in 2012, China banned Filipino fishermen from operating there. The ban was eased last month after Duterte visited Beijing to mend ties, with the Filipinos allowed to fish outside the lagoon.

A spokesperson for Duterte told AFP on Wednesday his office would soon release an executive order on the new "no-fishing zone" for both Filipinos and Chinese fishermen.

"This would be subject to friendly negotiations so we won't create friction with other claimants," said Ana Marie Banaag, presidential communications assistant secretary.

Chinese foreign ministry spokesperson Geng Shuang on Tuesday had no comment on Duterte's declaration but said Beijing had made "proper arrangements for fishing activities".

However Filipino fishermen told AFP the Duterte plan would hurt their livelihood.

"We are against that because it is inside the lagoon where there is more catch," said Charlito Maniago, village captain in Infanta, one of the main Scarborough Shoal fishing towns on Luzon.

Maritime law expert Jay Batongbacal said Duterte's declaration favoured China since the Philippines could further lose access to the shoal.

"China could just as easily accept the Philippine move and not act in return, because they derive the benefit anyway," Batongbacal said in a statement.

Following a case brought by Duterte's predecessor Benigno Aquino three years ago, a UN-backed international tribunal in July declared the shoal a common fishing ground for surrounding nations.

It also ruled that China's claim to most of the South China Sea was without legal basis, in a resounding legal victory for the Philippines.

But Duterte, 71, pivoted his country's diplomacy away from traditional ally the United States and towards China.

Soon after his ice-breaking trip to Beijing, Chinese vessels stationed at Scarborough allowed Filipino fishermen to fish outside the lagoon.