Protesters marching against China display placards during a rally over the South China Sea disputes in front of a Chinese Consulate in Makati city, metro Manila, Philippines August 6, 2016. Photo: Reuters / Romeo Ranoco

The Philippines went on the offensive on Tuesday to assert its sovereign rights over a fishing ground in the Pacific Ocean, stressing a need to protect it after a Chinese survey ship was monitored in the area last year.

Manila has lodged a diplomatic protest with Beijing after a Chinese vessel was tracked moving back and forth over Benham Rise, a vast area east of the Philippines declared by the United Nations in 2012 as part of its continental shelf.

President Rodrigo Duterte on Monday called for “structures” to be erected in the area to denote Philippine jurisdiction and told the navy that should Chinese vessels return, “go there and tell them straight that this is ours.”

The preemptive push by the Philippines underlines decades of mistrust of China over its activities in the South China Sea, which has been largely put aside since Duterte took office, who also sought to tap Beijing for business.

The foreign ministry said there was no question, and no rival claim, over Benham Rise, a 13 million hectare undersea region rich in biodiversity and yellow fin tuna.

“It is indisputable because no other country has an overlapping claim there,” foreign ministry spokesman Charles Jose said.

“So as a country that exercises sovereign rights and jurisdiction, we are the only one that has a sole and exclusive right to explore and exploit the national resources in Benham Rise. It is our responsibility to protect it.”

Philippine officials are suspicious and say the Chinese ship was not passing through the area, but surveying it.

Officially, there is no dispute over Benham Rise and statements by China in recent days have not challenged those of the Philippines about jurisdiction of the area.

China’s foreign ministry on Friday said the ship was engaged in “normal freedom of navigation and right of innocent passage,” and nothing more.

Philippine presidential spokesman Ernesto Abella said such passage was acceptable, “but they [China] are disallowed to stay and establish any structure,” he said.

“First and foremost, Benham Rise belongs to the Philippine people,” he added.

Former foreign minister Albert del Rosario, who has a history of challenging China, suggested the Philippines work with ally the United States to outline options for Manila to deal with Beijing over Benham Rise.

“Under no circumstances would it be wise for us to trade away our national security,” he said.