China and the Philippines have each unfurled their national flags on tiny sandbars in the South China Sea, staking competing sovereignty claims in strategic waters seen as a potential flashpoint for global conflict.

China’s official declaration of sovereignty over the land near the Philippines will raise fears that Beijing intends to build up unoccupied reefs and banks.

The competing claims are reigniting tensions in the South China Sea and coincide with joint military drills between the U.S. and the Philippines.