U.S. officials are taking seriously a string of provocative threats from the North Korean government, Defense Secretary Chuck Hagel said Thursday, hours after the U.S. military dispatched stealth planes capable of dropping nuclear-armed missiles for a training exercise in South Korea.

The sorties by B-2 bombers marked a rare show of force by the Pentagon on the Korean Peninsula and followed a decision this month to bolster nuclear defenses along the U.S. West Coast by adding 14 missile interceptors in Alaska.

“We have to take seriously every provocative, bellicose word and action,” Hagel told reporters during a news conference at the Pentagon on Thursday afternoon.

North Korea has used heated rhetoric for years, but its recent statements and actions have “ratcheted up the danger,” Hagel said, adding: “We have to understand that new reality.”

The U.S. military command in South Korea said in a statement that the B-2 sorties were meant to demonstrate “the commitment of the United States and its capability to defend” South Korea and other regional allies that are wary of nuclear-armed North Korea.

By dispatching one of the U.S. military’s most expensive and specialized aircraft from Whiteman Air Force Base in Missouri to drop inert munitions over South Korea’s Jik Do Range, the Pentagon sent a signal of how quickly and severely Washington could respond to an attack by Pyongyang. B-2 bombers cost $3 billion apiece, and flying them costs about $135,000 per hour, according to a study by the Center for Public Integrity.

North Korea has condemned the military training exercise as “warmongering.”

North Korean leader Kim Jong Un has in recent days threatened to strike U.S. military installations in South Korea and elsewhere in the Pacific. On Friday, in response to the B-2 bomber drills, he said his rocket forces are ready “to settle accounts with the U.S.,” the Associated Press reported.

In February, North Korea defied international appeals and conducted its third nuclear test.

North Korea cut off its military phone line with South Korea on Wednesday, suggesting that it no longer saw value in keeping an open line of dialogue. A statement carried by the North’s official Korean Central News Agency said that “not words but only arms will work on the U.S. and the South Korean puppet forces.”

In another statement, the news agency said South Korea’s military was behaving “like a puppy knowing no fear of a tiger.”

Hagel said the recent acts could represent an effort by North Korea’s 30-year-old leader to prod the West into making concessions, such as loosening the sanctions the United States and its allies have imposed on the isolated and secretive state.

“The fact is this is the wrong way to go,” Hagel said.