Tomahawk cruise missiles can carry conventional explosive warheads. Or nuclear ones.
"It's not unheard of to do that," a former senior defence official told Foreign Policy. "But I would say it is a fairly significant indicator that the possibility of using Tomahawks is rising."
A WARNO is essentially a battle-stations alarm: be prepared to take instant action.
But it does not necessarily mean Washington is preparing to attack Kim Jong-un.
It's a warning to be ready for anything, with fingers on triggers.
In the case of the Tomahawk cruise missile, it means the advanced weapon needs to be checked. Is it properly fuelled? Are its guidance computers - designed to fly through a window from its launch point hundreds of kilometres away after flying low through terrain to remain undetected - properly programmed?
Does it have any faults?
Does it have the appropriate warhead fitted?
"You would certainly want your Tomahawks ready in a fast-moving scenario like that should the president or secretary of defence make the judgement to respond with an offensive strike," FP quotes an unnamed former Pentagon official as saying.
"The fact that it's for a Tomahawk strike into [North Korea] feels ominous, but my guess it's more about having a quick strike response should 'Rocket Man' make an irrational decision and overt provocation," FP quotes former naval commander Ted Johnson as saying.
'BY MILITARY FORCE IF NECESSARY'
The revelation comes as CIA director Mike Pompeo warns that the United States should assume Kim Jong-un's regime is "on the cusp" of getting a nuclear missile capable of striking mainland targets.
The CIA head said President Donald Trump is determined to prevent North Korea from making such a military technological breakthrough "whether it happens on Tuesday or a month from Tuesday."
Both Pompeo and US National Security Adviser HR McMaster said Trump would still prefer to use sanctions and diplomacy to force Kim to come to the table to discuss disarmament.
But, speaking to a Washington policy forum, both also warned that the use of US military force remains an option to prevent Pyongyang from acquiring a long-range nuclear missile.
"They are close enough now in their capabilities that from a US policy perspective we ought to behave as if we are on the cusp of them achieving that objective," Pompeo said.
Pompeo said US intelligence had kept close tabs on the North Korean program in the past, but that its missile expertise is now growing too quickly to be sure when it will succeed.
"But when you're now talking about months our capacity to understand that at a detailed level is in some sense irrelevant," he said.
"The president's made it very clear," he added.
"He's prepared to ensure that Kim Jong-un doesn't have the capacity to hold America at risk. By military force if necessary."
This week, North Korea's deputy UN ambassador declared that Pyongyang would not put its nuclear arsenal nor ballistic missile program on the table unless Washington drops its "hostile" stance.
And Kim's regime has made no secret of its efforts to develop an intercontinental ballistic missile capable of hitting US mainland cities or bases in the Pacific, conducting regular tests.
McMaster told the conference, organised by the Federation for Defense of Democracies (FDD), that the regime would not be allowed to develop arms that would threaten the United States.
"We are not out of time but we are running out of time," Trump's top security adviser said.
"The president has been very clear. He's not going to accept this regime threatening the United States with nuclear weapons," he warned.
"There are those that say, 'accept and deter'. Well, 'accept and deter' is unacceptable."
