Washington (CNN) The US Air Force conducted two bomber flights this week into areas considered sensitive by the Chinese military, missions that have come amid heightened tensions between Washington and Beijing.

Earlier this week, US B-52 bombers flew from Guam and transited through the South China Sea, an area where the Chinese government has built islands and established military facilities on disputed features.

"That just goes on, if it was 20 years ago and had they not militarized those features there it would have been just another bomber on its way to Diego Garcia or wherever," Secretary of Defense James Mattis told reporters at the Pentagon Wednesday when asked about the bomber flight.

"There's nothing out of the ordinary about it," Mattis added.

On Tuesday, US B-52s also "participated in a regularly scheduled, combined operation in the vicinity of the East China Sea," Pentagon spokesman Lt. Col. Dave Eastburn told CNN.

Read More