Pyongyang has demonstrated its rockets are powerful enough to reach the United States, but General Paul Selva, vice chairman of the Joint Chiefs of Staff, said it has yet to prove that its fusing and targeting technologies can survive the stresses of ballistic missile flight.

"They have made some strides but it's still true that they haven't demonstrated all of the components of an intercontinental ballistic missile system," Selva told reporters.

Also unclear is whether North Korea has a re-entry vehicle strong enough to return into the Earth's atmosphere from space and deliver a warhead.

"It's possible (North Korean leader Kim Jong-Un) has them, so we have to place the bet that he might have them, but he hasn't demonstrated them," Selva said.

In an interview with the BBC, CIA Director Mike Pompeo warned that North Korea is rapidly closing the few remaining gaps before it has a deployable ICBM.

We "have developed a pretty clear understanding of Kim Jong-Un's capability," Pompeo said.

"We talk about him having the capability to deliver a nuclear weapon to the United States in a matter of a handful of months."

Last year, North Korea tested ICBMs that had the potential range to reach the US mainland and in September it conducted its sixth and largest nuclear test.

Pyongyang's weapons program has seen tensions on the Korean peninsula ratchet up in recent months, prompting fresh rounds of sanctions and fiery rhetoric from President Donald Trump and Kim.

At the United Nations in September, Trump vowed to "totally destroy" North Korea if it launches an attack on the United States.

Selva said North Korea has also become very good at predicting when spy satellites are overhead and adept at camouflaging its missiles.

Kim has developed new techniques of getting a missile to a launchpad, so the US and its allies might now only get about 12 minutes' warning before North Korea launched a missile, down from up to an hour previously, Selva added.