UN Secretary-General Ban Ki-moon attends the UN Academic Impact Seoul Forum in the South Korean capital on Wednesday. Ban said his plan to go to North Korea this week to visit an industrial complex had been scrapped after Pyongyang retracted its earlier approval.

AFP/Seoul

North Korea said on Wednesday it has the technology to make nuclear missile warheads, less than two weeks after announcing it had launched a ballistic missile from a submarine.

It also cancelled a visit by UN chief Ban Ki-moon, a day after he accused it of fuelling regional tensions.

"It has been a long time since we began miniaturising and diversifying our means of nuclear strike," Pyongyang's powerful National Defence Commission (NDC) said in a statement on the official news agency.

"We have also reached the stage where the highest accuracy rate is guaranteed, not only for short- and medium-range missiles but long-range missiles as well."

Earlier this month the North claimed it successfully test-fired a submarine-launched ballistic missile (SLBM), a development which - if confirmed - would allow the deployment of nuclear weapons far beyond the Korean peninsula.

Ban on Tuesday had urged the North to avoid any actions that could escalate military tensions.

A day later, he announced that Pyongyang had cancelled his invitation to visit the Kaesong industrial zone inside the North this week.

"No explanation was given for this last-minute change," the UN chief, a former foreign minister of South Korea, told a forum in Seoul.

‘Regrettable decision’

"This decision by Pyongyang is deeply regrettable," Ban said.

"However, I as the secretary-general of the UN, will not spare any efforts to encourage the DPRK (North Korea) to work with the international community for peace and stability on the Korean peninsula and beyond."

The NDC, which is chaired by leader Kim Jong-Un, said the SLBM test "represents a higher stage in our efforts to develop strategic striking forces" as part of strengthening its defences.

"The US and its cronies must stop kicking up a ruckus over our measures to strengthen self-defence capabilities," it said.

The North has conducted three atomic tests and has long had nuclear weapons. But it was unclear whether it possessed the technology to deliver them by missile.

However the US top homeland security commander, Admiral William Gortney, said last month the North is capable of mounting a miniaturised nuclear warhead on its new KN-08 intercontinental ballistic missile.

Ban had planned to travel on Thursday to the Kaesong industrial estate, a joint enterprise between Pyongyang and Seoul 10 kilometres over the border inside North Korea.

It hosts around 120 South Korean firms employing some 53,000 North Korean workers.

Had the visit gone ahead, Ban would have become the first UN secretary-general to set foot in the communist state for more than 20 years, since Boutros Boutros-Ghali in 1993.

But he was not expected to have any high-level talks during his brief visit.