The assessment has yet to become a formal consensus view of the U.S. government. But it reveals just how far along many in the U.S. believe the reclusive country has come to gaining a nuclear-tipped ballistic missile that could potentially strike the U.S.
As North Korean leader Kim Jong Un's public rhetoric has escalated in recent weeks, concern has grown inside intelligence circles that he has made progress on several fronts.
"He is determined to prove his doubters wrong," one U.S. official told CNN, even as uncertainty remains about how much progress he has actually made in his quest for nuclear missiles.
Recent photos showing Kim standing next to what the North Koreans claim is a miniaturized nuclear device are still being scrutinized by U.S. analysts for any indication of progress, officials said, declining to provide additional specifics.
U.S. officials who endorse the notion that Kim probably has a nuclear warhead still note that they don't know if the device would actually work. The North Koreans believe it would.
U.S. commanders have said they assume for war planning purposes that North Korea has a functional warhead but have stopped short of outright declaring it exists.
"It's the prudent decision on my part to assume that he has the capability to miniaturize a nuclear weapon and put it on an ICBM," Adm. William Gortney, head of U.S. Northern Command, recently told Congress.
But the Pentagon has taken pains to downplay the possibility.
Pentagon Press Secretary Peter Cook acknowledged that "the commanders who are responsible for these activities" are "doing the prudent, appropriate, proper" thing by assuming the North Koreans possess this capability.
But he said that this operational assumption "does not mean that they have that capability. They've not demonstrated that."
At the same time, the U.S. believes North Korea is making progress in testing an advanced version of its KN-08 intercontinental ballistic missile.
North Korean state media released photos Thursday of Kim inspecting a test of a new solid-fuel rocket engine, a key component of the KN-08 missile. The U.S. officials have seen pictures and for now don't have reason to doubt their authenticity.
There are also indications that North Korea might be preparing for yet another underground nuclear test, though one official said that specific signs of preparation at the test site itself are not yet visible.
The North Korean military's last nuclear test was in January, a test they claimed was a hydrogen bomb detonation. U.S. officials disputed that claim.