MANILA—China has begun a construction project in the disputed Spratly Islands, drawing a formal protest from the Philippines, which said the move violates a longstanding pledge not to build on any of the South China Sea's contested islands.

The project is the latest example of Beijing's increasingly assertive behavior in the South China Sea, a week after it boldly deployed a large oil rig, guarded by a flotilla of ships, to disputed waters east of Vietnam.

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