The unprecedented U.S. decision to send nuclear-capable B-2 stealth bombers to drop dummy munitions during military drills with South Korea this week was part of normal exercises and not intended to provoke a reaction from North Korea, Defense Secretary Chuck Hagel said Thursday.

Hagel acknowledged, however, that North Korea's belligerent tones and actions in recent weeks have ratcheted up the danger in the region, “and we have to understand that reality.”

Speaking to Pentagon reporters, both Hagel and Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said the B-2 bombers were a message intended more for allies than Pyongyang.

“The North Koreans have to understand that what they're doing is very dangerous,” Hagel said. “I don't think we're doing anything extraordinary or provocative or out of the ... orbit of what nations do to protect their own interests.” The U.S., he added, must make it clear to South Korea, Japan and other allies in the region that “these provocations by the North are taken by us very seriously and we'll respond to that.”

U.S. Forces Korea announced in a statement Thursday that two B-2 stealth bombers flew from an air base in Missouri and dropped dummy munitions on a South Korean island range before returning home. While B-2 bombers have been used in past military exercises, including one in 2000 that included flights over South Korea, this is the first time that dummy munitions were dropped, according to the Pentagon.

The joint drills are likely to heighten the already escalating tensions between the U.S. and North Korea that have played out in recent weeks, including Pyongyang's threat to carry out nuclear strikes on Washington and Seoul. North Korea has ramped up its rhetoric in response to the recent U.S. military exercises and also the U.N. sanctions over North Korea's nuclear test last month.

Use of the stealthy B-2 bombers added something of an exclamation point to the training mission, which had already included older but also nuclear-capable B-52 bombers.

“They're telling the North Koreans, we can attack you in ways in which you can see us coming, and we can also attack you potentially in ways in which you cannot see us coming,” said retired Air Force Col. Cedric Leighton, a North Korean intelligence expert who served on the Joint Staff and the National Security Council. “So it's a message to the North Koreans that they have to be very careful how they proceed next with their military efforts and their political efforts.”

Asked if the U.S. has seen North Korea take any actual threatening military steps in response to the bombers, Dempsey said the North has moved some artillery units across the demilitarized zone from Seoul and some maritime units along the coasts. But so far, he said, “We haven't seen anything that would cause us to believe they are movements other than consistent with historic patterns and training exercises.”

The military drills are only the latest U.S. response to what officials see as a growing North Korean threat. The Pentagon is also planning to beef up its defenses against a potential North Korean missile attack on the U.S.

Hagel announced earlier this month that over the coming four years the Pentagon will add 14 missile interceptors to the 26 it already has in place at a base in Alaska, at an estimated cost of $1 billion.

Asked about the cost of sending the B-2 bombers all the way from the U.S. to South Korea for a show of force, Dempsey said the military plans for a certain number of exercises each year involving the B-2 and B-52 bombers.

Even if it wasn't in the budget, he said, “in light of what's happened in North Korea and the provocation and the necessity of assuring our allies that we're there with them, we would have found a way to do this.”

Hagel said there are a lot of “unknowns” with North Korea and its new president Kim Jong Un.

“But, we have to take seriously every provocative, bellicose word and action that this new, young leader has taken so far since he's come to power,” Hagel said.

Leighton and other experts say that a full-blown North Korean attack is not likely. But there are persistent worries about a more localized conflict, such as artillery attacks or a naval skirmish in the disputed Yellow Sea waters. There have been three naval clashes since 1999.

“You may see some shelling of South Korean islands that are very close to the North Korean coast.  They've done that in the past, they killed four people the last time they did this. That could happen again,” said Leighton. (AP)