North Korea will pursue "acceleration" of its nuclear and missile programmes including developing a "pre-emptive first-strike capability", a diplomat from Pyongyang warned on Tuesday.

Choe Myong-nam, deputy ambassador of the North Korean mission to the United Nations in Geneva, told Reuters news agency that US threats of increased sanctions would not faze the country.

"I think this is stemming from the visit by the Secretary of State [Rex Tillerson] to Japan, South Korea and China … We of course are not afraid of any act like that," Choe said.

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"Even prohibition of the international transactions system, the global financial system, this kind of thing is part of their system that will not frighten us or make any difference."

He called existing sanctions "heinous and inhumane".

North Korea has been under sanctions for "half a century", but the communist state survives by placing an emphasis on juche or "self-sufficiency", he said.

His country wants an international forum set up to examine the "legality and legitimacy of the sanctions regime".

Choe denounced joint annual military exercises currently being carried out by the United States and South Korea on the divided peninsula, and criticised remarks by Tillerson during his talks with regional allies last week.

"All he was talking about is for the United States to take military actions on DPRK [the Democratic People's Republic of Korea]," Choe said.

North Korea rejects claims by Washington and Seoul that the military drills are defensive. They involve strategic nuclear bombers and a nuclear submarine Columbus that recently entered South Korean ports, he said.

"In the light of such huge military forces involved in the joint military exercises, we have no other choice but to continue with our full acceleration of the nuclear programmes and missile programmes," said Choe.

"It is because of these hostile activities on the part of the United States and South Korea we strengthen our national defence capability, as well as pre-emptive strike capabilities with nuclear forces as a centrepiece."

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He declined to give technical details of North Korea's latest rocket engine test on Sunday - seen as a possible prelude to a partial inter-continental ballistic missile flight - calling it a great historical event that would lead to "fruitful outcomes".

"I can tell you for sure that the inter-continental ballistic rockets of the DPRK will be launched at any time and at any place as decided by our supreme leadership," Choe said, recalling leader Kim Jong-un's pledge in a New Year's address.

Analysts say North Korea has likely mastered the technology to power the different stages of an ICBM and may show it off soon, but is likely to be still a long way from being able to hit the mainland United States.

"The United States has been talking about launching pre-emptive strikes at North Korea and we have been prepared to deter, to counter-attack such attacks," Choe said.

"We would utilise every possible means in our hands and the inter-continental ballistic rocket is one of them."