The U.S. military sent two B-52 heavy bombers for a "routine mission" in the contested South China Sea on Tuesday, just days after Defense Secretary Jim Mattis lambasted Beijing's continued militarization of its outposts in the strategic waterway.

The U.S. Pacific Air Forces, which oversees operations in the region, told The Japan Times that the two nuclear-capable bombers departed a navy facility on Diego Garcia in the Indian Ocean for training "in the vicinity of the South China Sea" prior to returning to Diego Garcia.

The exact location of the training mission was unclear, as the U.S. military does not discuss specifics for operational security reasons, but Aircraft Spots, a Twitter account that tracks aircraft and U.S. Air Force flights, tweeted a photo of a flight path that showed the bombers had passed near or over the disputed Spratly island chain.