Experts believe North Korea is probably 6-12 months away from developing an ICBM fitted with a nuclear warhead that will work more often than it fails.
While US intelligence officials are pretty sure North Korea can put a nuclear warhead on an intercontinental missile that could reach the United States, experts aren't convinced it would survive the flight.
They cite lingering questions about Kim Jong Un's nuclear know-how.
Could North Korea deploy nuclear weapons successfully time after time and hit their intended targets?
Would its weapon system break apart from the heat and stress it would sustain as it re-enters the atmosphere roughly 10 times faster than a speeding bullet?
"I don't think North Korea has a good measure of how accurate the missile is at this point," said Michael Elleman, an expert with the International Institute for Strategic Studies.
"They don't know if the re-entry technologies will really hold up, whether the bomb will survive the trip."
North Korea has short-range missiles that can hit its neighbours.
It has tested an intermediate one that could strike Guam, a US territory, as well as a longer-range missile that could reach Hawaii and perhaps the West Coast of the United States.
The intermediate and long-range missiles are still being developed and it's still questionable if they can reliably strike targets.
Kim must do more tests to master what is known as "re-entry" in missile parlance, experts believe.
The process involves shielding a nuclear warhead from the high temperatures and force it faces upon re-entry at 7 kilometers a second.
"In principle, Kim Jong Un could hit the United States with a nuclear weapon," said Elleman, a former scientist at Lockheed Martin's Research and Development Laboratory who also worked as missile expert for UN weapons inspection missions.
"In practice, I think they are probably a half-year to a full year away from having something that will work more often than it would fail."
Joseph Bermudez, an internationally recognised expert on North Korean defense and intelligence affairs and ballistic missile development, agrees.
"Putting these things all together and making them work is extremely challenging and they haven't yet demonstrated a capability to produce a reliable re-entry vehicle, which is what houses the actual nuclear device," he said.
"Remember, they've only tested these systems very few times."
Still, Bermudez, said, North Korea is "on track" to figure it out.
US officials also think it's just a matter of time before Kim's program fully matures.
National Intelligence Director Dan Coats told Congress in May Kim has been photographed beside a nuclear warhead design and missile airframes to show that North Korea has warheads small enough to fit on a missile.
North Korea conducted its first test of an intercontinental missile on July 4. On July 28, it conducted a second test of its long-range Hwasong-14 ICBM.
The second test flight was captured by a rooftop camera operated by Japan's NHK television on the northern island of Hokkaido.
Elleman, who analysed the video, concluded it most likely "disintegrated" before splashdown, suggesting North Korea is still struggling with re-entry.