Washington, Sep 27 (IANS): US bombers flew from Guam and transited through the South China Sea, an area considered sensitive by the Chinese military.

"That just goes on, if it was 20 years ago and had they not militarised those features there it would have been just another bomber on its way to Diego Garcia or wherever," Secretary of Defence 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, the 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.

A US defence official said that the bombers were escorted by Japanese fighter jets and flew in proximity to the Japanese controlled Senkaku Islands which China lays claim to.

The bombers also flew into the Chinese military's unilaterally declared Air Defence Identification Zone which extends over the area.

The two missions comes amid heightened tensions over a series of issues.

In the last week, the Chinese government denied a US Navy warship permission to visit Hong Kong, the US sanctioned a Chinese defence entity over its purchase of Russian-made weapons, the State Department approved a military equipment sale to Taiwan and a high-ranking Chinese naval officer cancelled a meeting with his American counterpart.

"We're sorting out obviously a period with some tension there, trade tension and all, so we'll get to the bottom of it but I don't think that we're seeing a fundamental shift in anything, we're just going through one of those periodic points where we got to learn to manage our differences," Mattis said on Wednesday when asked about the tensions.