Washington (CNN) China has placed artillery pieces on one of the islands it is manufacturing in contested waters in the South China Sea, a U.S. official told CNN Friday. The story was first reported by The Wall Street Journal.

The official added that the artillery pieces are no longer visible -- having either been removed or concealed -- following a U.S. Navy P8-A Poseidon surveillance mission over the area. CNN was on board the flight, which took place last week.

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Amidst the Chinese military buildup, Washington and Beijing are engaging in a war of words over the disputed South China Sea territory, with pledges of military maneuvers in the area.

"China urges the U.S., think three times before taking action; act responsibly; stop all provocative speech and acts; and do more things that will benefit the region's peace and stability instead of the opposite," Foreign Ministry spokesperson Hua Chunying said Thursday.

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