President Duterte has ordered the Philippine Navy to put up “structures” to assert the country’s sovereignty over an underwater landmass 250 kilometers off the east coast of Luzon that a Chinese survey ship visited for six months last year, according to Defense Secretary Delfin Lorenzana.

Manila has lodged a diplomatic protest with Beijing after the Chinese vessel was tracked moving back and forth over Benham Rise, which the United Nations declared in 2012 part of the Philippine continental shelf.

According to the Philippine Daily Inquirer reports, the 13-million-hectare Benham Rise, also known as Benham Plateau, is found off Isabela and Aurora provinces.

Although the United Nations ruled in favor of the Philippines, this did not mean Benham Rise is part of its territory, according to Chinese foreign ministry spokesperson Geng Shuang.

Lorenzana said Pres. Duterte’s instruction was to increase naval patrols in that area and put up structures “that say this is ours.”