North Korea's nuclear weapons capabilities and ballistic missile defence programmes constitute a "serious threat" to the United States, Defense Secretary Ash Carter has said.

The US was prepared to shoot down a North Korean missile launch or test "if it were coming towards our territory or the territory of our friends and allies," Mr Carter said during an appearance on NBC's "Meet the Press."

It comes as the US reported the secretive dictatorship had demonstrated a "qualitative" improvement in its nuclear and missile capabilities following an unprecedented increase in the number of tests it performs.

Experts have said Pyongyang has been testing rocket engines and heat-shields for their intercontinental ballistic missiles (ICBMs) and developing the technology to guide it towards a target as it re-enters the atmosphere after take-off.

On New Year's Day, the country claimed it could test launch an intercontinental missile at any time from any location set by leader Kim Jong-un, saying a hostile US policy is to blame for its arms development.

An unnamed Foreign Ministry spokesman told the state news agency, KCNA news: "The US is wholly to blame for pushing the DPRK to have developed ICBM as it has desperately resorted to anachronistic policy hostile toward the DPRK for decades to encroach upon its sovereignty and vital rights.

"Anyone who wants to deal with the DPRK would be well advised to secure a new way of thinking after having clear understanding of it".

While Pyongyang is close to a sixth test, it is likely to take some years to perfect the weapon, according to the experts.

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Once fully developed, a North Korean ICBM could threaten the continental US, which is around 9,000 km (5,500 miles) away. ICBMs have a minimum range of about 5,500 km (3,400 miles), but some are designed to travel 10,000 km (6,200 miles) or further.

North Korea has been subject to UN economic sanctions since 2006 over its nuclear and ballistic missile tests. The sanctions were tightened in the last year after the country carried out its fifth and largest test in September.

But many believe the country will only back down from its current programme of nuclear development if its ally China intervenes.

Beijing is believed to be reluctant to rein in its smaller neighbour, which it props up with millions in aid every year, as it fears an influx of Korean refugees on its southern border if the Kims were to be toppled.