North Korea has developed the capability to pair a nuclear warhead with a missile and launch it, but has not mastered bringing the weapon back from space and onto a target, a senior US defense official said Thursday.

Pyongyang has conducted a series of missile launches in the wake of its fourth nuclear test in January, to the consternation of regional countries and many in the international community.

While experts say the North is thought to have succeeded in making nuclear warheads small enough to arm Scud missiles, it is unclear if they can put a weapon on a bigger rocket that travels further and deploys a warhead from space.

"Truthfully, they have the capability right now to be able to deliver a nuclear weapon, they are just not sure about re-entry, that's why they continue to test their systems out there," the official said.

He added that he believed the North Koreans can already "mate" a missile with a warhead.

But "they are not sure of the re-entry capability for a strategic strike, so they are endeavoring to try and overcome that."

In the wake of ramped-up testing by North Korea's leader Kim Jong-Un, the United States is deploying an advanced missile defense system in South Korea.

Despite strong objections from China and Russia, the Terminal High Altitude Area Defense (THAAD) system will be ready within about 10 months, the Pentagon says.

North Korea's continued nuclear testing is generating renewed concern in the US military, and the Pentagon has numerous contingency plans to more assertively try to rein in the country's atomic capabilities.

Something we are "very much leaning into (is) being more prepared for the future (in) North Korea," the official said.

"It is the threat that keeps me awake at night. You've heard other senior leaders say the same thing, primarily because we don't know what the 'Dear Leader' in North Korea really is after."

The official noted that the United States and its allies have little leverage over isolationist North Korea.

"We are in a very tenuous situation with not a lot of leverage, not a lot of initiative in terms of negotiations," the official said.

"So as you might imagine, we are preparing for contingency operations to the degree we need to."

The official said the United States has had backup military plans at the ready since the 1953 armistice between the North and South.