Chinese navy and coast guard vessels nearly doubled their presence last month in parts of the South China Sea claimed by the Philippines, Manila said Thursday. Philippine authorities track the ships as they move through reefs and shoals inside waters the country asserts as its own. Beijing maintains a claim over most of the strategic waterway, an argument that a 2016 international arbitral ruling rejected as legally baseless. Encounters between the two sides' ships have repeatedly turned tense. Water cannon blasts and deliberate blocking maneuvers have marked past standoffs near contested features. The spike in vessel numbers fits a longer pattern of China expanding its footprint in the area. Filipino fishers and patrol crews keep working these waters under watchful eyes on both sides.