WASHINGTON, Dec 18 (Reuters) - A U.S. B-52 bomber on a routine mission mistakenly flew within 2 nautical miles of a Chinese-built artificial island in the South China Sea last week, the Wall Street Journal reported on Friday, citing the Pentagon.

One of two planes on the mission flew closer than planned to Cuarteron Reef in the Spratly Islands, the Journal said, adding that the Pentagon was investigating the reasons why the bomber came so close to the reef.

China and its neighbors have competing claims over the Spratlys, and the issue has become a source of friction between Beijing and Washington.

(Reporting by David Alexander; Editing by Mohammad Zargham)

Our Standards: The Thomson Reuters Trust Principles.