The Pentagon on Friday repeated reassurances that the United States and its regional allies are well protected against any threat from North Korean missiles, after Pyongyang conducted a new nuclear test.

The renegade country has now conducted five nuclear tests and dozens of missile launches, and says it has realized its goal of being able to fit a miniaturized warhead on a rocket.

"This is a threat we recognize as very real and we are always ensuring that we are outpacing it," Pentagon spokesman Captain Jeff Davis said.

"We take them at their word and have built a defensive architecture that assumes that they have the capability -- but we have never actually seen it demonstrated."

The United States has a range of missile-defense technologies at its disposal, including the Aegis Combat System, Patriot missiles and sophisticated radars.

Washington is also working with Seoul to install a missile defense system known as THAAD in South Korea, though the move has infuriated Beijing, which says the hardware poses a significant threat to regional security.

Davis said the THAAD system would be up and ready in 2017, though it could be installed quicker in an emergency.

"There is a lengthy process we are working through with (Seoul) but of course... we can always move it much more quickly," he said.

The US Air Force will fly a specially equipped WC-135 jet that can detect radiation and other particles in the sky to determine what happened in North Korea.

"Certainly all the indications we've seen based on the seismic activity, it's certainly consistent with a nuclear test," Davis said.