All four reefs were already occupied by Chinese forces but are also claimed by the Philippines.
China claims the Spratly Islands along with nearly all of the South China Sea, which contains vital sea routes and is also believed to hold large mineral resources.
The Philippines, along with Vietnam, Brunei, Malaysia and Taiwan have conflicting claims to parts or all of the same territory, which has led to tense confrontations in recent years.
In recent weeks, China and Vietnam have traded accusations of their ships ramming each other after China set up an oil rig in a South China Sea area also claimed by Vietnam.
The Philippines asked a United Nations tribunal in March to declare China's claim to most of the South China Sea illegal.
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