WASHINGTON (AP) - The unprecedented US 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, Defence Secretary Chuck Hagel said.
Mr Hagel acknowledged, however, that North Korea's belligerent tones and actions in recent weeks have increased the danger in the region, "and we have to understand that reality".
North Korea's leader said on Thursday that his rocket forces are ready "to settle accounts with the US" in response to the B-2 bombers. State media said leader Kim Jong Un ordered rockets on standby to strike the US mainland, South Korea, Guam and Hawaii.
Speaking to reporters earlier, both Mr Hagel and General 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," Mr 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 United States, 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".
US Forces Korea announced in a statement on Thursday that two B-2 stealth bombers flew from an air base in the US and dropped dummy munitions on a South Korean island range before returning home. The Pentagon said this was the first time dummy munitions had been dropped over South Korea, but it was unclear if there ever had been any B-2 flights in South Korea.
The joint drills are likely to heighten the escalating tensions between the US and North Korea in recent weeks, including Pyongyang's threat to carry out nuclear strikes on Washington and Seoul. North Korea is also angry at new United Nations sanctions over its Feb 12 nuclear test last month.
Asked if the US has seen North Korea take any actual threatening military steps in response to the bombers, Gen Dempsey said the North has moved some artillery units across the demilitarised 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 US response to what officials see as a growing North Korean threat. The Pentagon is also planning to strengthen its defences against a potential North Korean missile attack on the US.
Mr 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 US$1 billion (S$1.2 billion).
Mr Hagel said there are a lot of "unknowns" with North Korea and its leader Kim.
"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," he said.
Experts say a full-blown North Korean attack is not likely. But there are persistent worries about a more localised conflict, such as artillery attacks or a naval skirmish in the disputed Yellow Sea waters. There have been three naval clashes between North and South Korea 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 retired Air Force Colonel Cedric Leighton, a North Korean intelligence expert who served on the Joint Staff and the National Security Council.