Philippine President Rodrigo Duterte is negotiating an agreement with China to share oil and natural-gas resources in the disputed waters of the South China Sea, a deal that would be a major policy victory for Beijing.
It would open the door for China to push for similar arrangements with other Southeast Asian nations that have challenged its expansive claims and potentially lock Western oil companies out of the resource-rich region.
Mr. Duterte’s opponents and legal experts say joint development—which Beijing has long advocated—legitimizes China’s assertion that it has historic rights over almost the entire South China Sea, undermining a 2016 international tribunal ruling that invalidated those claims. It would also fortify Beijing’s control over the strategic waters, where tensions between the U.S. and China have surged.