Washington (CNN) Two US B-52 bombers flew within the vicinity of the contested Spratly Islands in the South China Sea on Tuesday, according to a statement from US Pacific Air Forces, which oversees air operations in the region.

The flyover came days after Secretary of Defense James Mattis called Beijing out over its militarization of the islands, accusing China of "intimidation and coercion" in the Indo-Pacific, making clear the US has no plans to leave the region and prompting a furious Chinese response.

Beijing claims the Spratly Islands, but those claims aren't recognized by the US or by China's neighbors -- Malaysia, Vietnam, the Philippines and Taiwan -- which also say the islands are theirs. China has used geographic features in the Spratlys to build man-made islands, some of which it has equipped with military facilities, including anti-ship and anti-aircraft missiles.

A US defense official who has knowledge of the mission's original flight plan said it called for the nuclear capable B-52 bombers to fly about 20 miles from the islands.

A spokesman for the Pentagon said the mission involved the Guam-based bombers conducting "a routine training mission," flying from Andersen Air Force Base in the US territory of Guam "to the Navy Support Facility" in the United Kingdom's Indian Ocean territory of Diego Garcia.

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