North Korea says it can test-launch an intercontinental ballistic missile (ICBM) at any time from any location chosen by leader Kim Jong-un, adding that the United States' hostile policy was to blame for its arms development.

Kim said on January 1 that his nuclear-capable country was close to test-launching an ICBM.

"The ICBM will be launched any time and anywhere determined by the supreme headquarters of the DPRK," an unnamed foreign ministry spokesman was quoted as saying on Sunday by the official KCNA news agency, using the acronym for the country's name.

READ MORE: Analysts say North Korea not bluffing on ICBM launch

The North is formally known as the Democratic People's Republic of Korea.

Ashton Carter, the US defence secretary, said on Sunday that North Korea's nuclear weapons and ballistic missile programmes constitute a "serious threat" to the US.

Washington is 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", Carter said during an appearance on NBC's Meet the Press.

The US said on Thursday that North Korea had demonstrated a "qualitative" improvement in its nuclear and missile capabilities after an unprecedented level of tests last year.

Analysts have said while Pyongyang may be close to testing an ICBM, it would likely take years to perfect the weapon.

Once fully developed, a North Korean ICBM could threaten the continental US, which is about 9,000km from the North.

US president-elect Donald Trump responded last week to Kim's comments on an ICBM test by declaring in a tweet, "It won't happen!"

READ MORE: Kim Jong-un says close to testing long-range missile

North Korea has been under UN sanctions since 2006 over its nuclear and ballistic missile tests. The sanctions were tightened last month after Pyongyang conducted its fifth and largest nuclear test on September 9.

"The US is wholly to blame for pushing the DPRK to have developed ICBM as it has desperately resorted to an anachronistic policy hostile toward the DPRK for decades to encroach upon its sovereignty and vital rights," KCNA quoted the spokesman as saying.

"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."