For the first time since the Cold War, the U.S. is planning to put nuclear-armed bombers back on 24-hour alert, following the North Korea crisis.

"The world is now a dangerous place, and today there are people speaking openly about nuclear weapons," U.S. Air Force Gen. David Goldfein said in an interview released on Sunday.

"Now there are other players with nuclear competence," Goldfein said, pointing out that the world is not a bipolar place where the main actors are only the U.S. and the Soviet Union.

Goldfein said that at least one of the four E-4B Nightwatch planes, also known as "Apocalypse," is always ready for 24 hours in the U.S. government's emergency air fleet.

U.S. President Donald Trump spoke on Sunday about North Korea, saying that the government was "prepared for everything."

The U.S. Air Force began flying the B-52s around the Korean peninsula after North Korea's nuclear test in early 2016.

Meanwhile, Japan's Defense Minister Itsunori Odonera asserted yesterday that North Korea's nuclear and ballistic missile capabilities have grown to an "unprecedented, critical and imminent" level.

Odonera said that this rising threat means his country, along with South Korea and the United States, have to collectively take what he called "different responses."

His comments, made through an interpreter, came at the outset of a meeting in the Philippines with U.S. Defense Secretary Jim Mattis and South Korea's Defense Minister Song Young-moo.

Onodera noted that North Korea has recently launched ballistic missiles that have overflown Japanese territory and said it cannot be ruled out that a recent underground nuclear test by North Korea was a hydrogen bomb.

"The country has steadfastly improved it nuclear and missiles capability," he said. "The threat posed by North Korea has grown to the unprecedented, critical and imminent level."

"Therefore, we have to take calibrated and different responses to meet that level of threat," he added without elaborating on what "different" responses Japan favors.

Mattis was somewhat more reserved in his remarks, although he did slam Pyongyang for defying U.N. Security Council resolutions against its nuclear and ballistic missile programs. However, he did not mention any potential military action.

"North Korea's provocations threaten regional and global security," the U.S. defense secretary said, emphasizing a unified U.S.-Japan-South Korea position in pressuring the North to give up its nuclear program.