MANILA—The U.S. will start stationing warplanes in the Philippines this week as the vanguard of a major deployment to the Southeast Asian country as Washington and its allies mount a coordinated response to Beijing’s assertiveness in the South China Sea.

The U.S. and the Philippines began joint patrols of the South China Sea last month, Defense Secretary Ash Carter said Thursday on a visit to the Philippines, a U.S. treaty ally that is among the five governments whose territorial claims overlap with China’s in those waters.

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