The US Air Force is preparing to put nuclear bombers back on 24-hour ready alert for the first time since the Cold War.
B-52 planes loaded with nuclear weapons would be positioned to take off at any moment, with crews on standby at a base in Louisiana.
COMPLETE COVERAGE: North Korea vs US
The move comes amid rising tensions between the US and North Korea, with President Donald Trump saying that Washington is "prepared for anything" when it comes to Kim Jong-un's regime.
There are fears that a new war could break out as the hermit state tries to develop a nuclear warhead capable of hitting the US mainland.
Gen David Goldfein, the US Air Force chief of staff, told Defense One that his branch was preparing to put the B-52 bombers back on 24-hour ready alert for the first time since the Cold War ended in 1991.
He downplayed any suggestion that it was due to any particular conflict, such as tensions with North Korea.
The general said: "This is yet one more step in ensuring that we’re prepared.
"I look at it more as not planning for any specific event, but more for the reality of the global situation we find ourselves in and how we ensure we’re prepared going forward."
Gen Goldfein, a member of the Joint Chiefs of Staff which advises Trump, added that the alert order had not been given, but that the US Air Force was preparing for it.
Efforts are already underway to prepare Barksdale Air Force Base, outside Shreveport, Louisiana, for the new status, Defense One editor Marcus Weisgerber reported after touring the site.
They include a refurbished building to house more than 100 crew members.
Introduced in 1955, the Boeing B-52 Stratofortress was built to carry nuclear weapons and serve as a deterrent during the Cold War.
The long-range strategic bomber can carry 70,000lbs of nuclear and conventional weapons, including bombs, smart weapons, mines and missiles.
With eight turbojet engines, it has a top speed of 650mph and a range of 8,800 miles.
It has been hailed by manufacturer Boeing as the "most combat capable bomber" in the US forces' arsenal.
Meanwhile, North Korea has warned Trump that he is "taking the US into ruin" and "playing with fire" by moving combat assets to the region.
The president was called a "lunatic" and "hooligan" with "war fever" as a propaganda newspaper fired the latest salvo in the war of words between Pyongyang and Washington.
The Pyongyang Times, a state-owned weekly newspaper, responded after the US moved the USS Ronald Reagan nuclear aircraft carrier and fighter jets to South Korea.
In an interview with Fox News, Trump boasted that the US is "prepared for anything" amid the North Korea nuclear crisis.
The president added: “We’ll see what happens. We are so prepared, like you wouldn’t believe."
Amid calls for all sides to back down, ex-US President Jimmy Carter has said he would be willing to travel to North Korea on behalf of the Trump administration to help diffuse the situation.
Carter, a Nobel Peace Prize winner in 2002, told the New York Times: "I would go, yes."