Defense Secretary Chuck Hagel says the addition of nuclear-capable B-2 stealth bombers to military drills with South Korea was part of normal exercises and not intended to provoke a reaction from North Korea.

Hagel said Thursday that North Korea’s belligerent tones and actions in recent weeks have ratcheted up the danger in the region. He said the United States has to be prepared to defend its interests as well as the interests of its allies.

The defense secretary says there are a lot of “unknowns” with North Korea.

U.S. Forces Korea said in a statement Thursday that two B-2 stealth bombers flew from an air base in Missouri and dropped dummy munitions on the South Korean island range before returning home.