North Korea has the capability of attaching miniaturised nuclear warheads to small missiles, but it is still having problems with missile re-entry technology for longer range strikes, claims a US defence department official.

The official said the communist state, officially known as the Democratic People's Republic of Korea (DPRK), can now build nuclear missiles with a maximum range of around 700 kilometres (430 miles).

However, Kim Jong-Un's regime does not yet boast larger inter-ballistic missiles capable of reaching targets beyond East Asia, as test weaponry continues to disintegrate upon re-entry from space.

"I think they could mate a warhead with a delivery device. They're just not sure about re-entry," Reuters quoted the official as saying.

"However, they're endeavouring to overcome that," he added.

"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 reclusive nation has conducted at least four nuclear tests and missile tests in defiance of United Nations resolutions over the past decade. Earlier this year North Korea proclaimed a breakthrough, stating that it had produced "miniature" nuclear warheads.

The US, acting partly in response to the threat, has confirmed it will deploy its Terminal High Altitude Area Defense (THAAD) system in South Korea in 2017, dismissing protests by China. It is also improving its missile defence systems in Alaska and California to shoot down any incoming missiles in mid-flight.

Despite the DPRK's apparent weaponry advances, the country is known to make grand military claims as part of its international bargaining strategy, typified by claims it manufactured a fully-functioning hydrogen bomb, that were rejected by experts.

North Korea nuclear threat
North Korean leader Kim Jong-un (C) smiles as he guides a test fire of a new multiple launch rocket system in this undated photo KCNA via Reuters