Hackers linked to China’s government broke into one of the U.S. government’s most sensitive computer networks, breaching a system used by the White House Military Office for nuclear commands, according to defense and intelligence officials familiar with the incident.

One official said the cyber breach was one of Beijing’s most brazen cyber attacks against the United States and highlights a failure of the Obama administration to press China on its persistent cyber attacks.

Disclosure of the cyber attack also comes amid heightened tensions in Asia, as the Pentagon moved two U.S. aircraft carrier strike groups and Marine amphibious units near waters by Japan’s Senkaku islands.

China and Japan—the United States’ closest ally in Asia and a defense treaty partner—are locked in a heated maritime dispute over the Senkakus, which China claims as its territory.