The White House warned Thursday that China would face "consequences" for its military buildup in contested waters in the western Pacific.
"We're well aware of China's militarisation of the South China Sea," said press secretary Sarah Sanders.
"We've raised concerns directly with the Chinese about this and there will be near-term and long-term consequences."
Sanders did not say what the consequences would be.
The South China Sea -- which sits between Vietnam, the Philippines, China and several other countries -- is subject to a myriad of competing territorial claims.
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But in recent years China has upped the ante, seizing islets and atolls in the face of protests and bellicose warnings.
Beijing Thursday reasserted its right to build "defense" facilities in the disputed region, but declined to confirm reports it had installed new missiles on artificial islands it had built.
Beijing sees the area as key to pushing its defenses beyond China's coast and securing oil supply routes.
Washington and other western countries have insisted that disputes must be settled legally and that freedom of navigation be respected.
