North Korea has threatened South Korea with "final destruction" during a debate at the United Nations Conference on Disarmament, saying it could take further steps after a nuclear test last week.

"As the saying goes, a new-born puppy knows no fear of a tiger. South Korea's erratic behaviour would only herald its final destruction," North Korean diplomat Jon Yong Ryong told the meeting.

Jon's comments drew quick criticism from other nations, including South Korea, France, Germany and Britain, whose ambassador Joanne Adamson said such language was "completely inappropriate" and the discussion with North Korea was heading in the wrong direction.

"It cannot be allowed that we have expressions which refer to the possible destruction of UN member states," she said.

Spanish Ambassador Javier Gil Catalina said the comment left him stupefied and appeared to be a breach of international law.

"In the 30 years of my career I've never heard anything like it and it seems to me that we are not speaking about something that is even admissible, we are speaking about a threat of the use of force that is prohibited by Article 2.4 of the United Nations charter," Catalina said.

Since the North tested a nuclear bomb last week in defiance of UN resolutions, its southern neighbour has warned it could strike the isolated state if it believed an attack was imminent.

Pyongyang said the aim of the test was to bolster its defences given the hostility of the United States, which has led a push to impose sanctions on North Korea.

"Our current nuclear test is the primary countermeasure taken by the DPRK in which it exercised its maximum self-restraint," said the North Korean diplomat Jon.

"If the US takes a hostile approach toward the DPRK to the last, rendering the situation complicated, it (North Korea) will be left with no option but to take the second and third stronger steps in succession," he said, without indicating what that might entail.

North Korea has already told key ally China that it is prepared to stage one or two more tests this year to force the United States into diplomatic talks, a source with direct knowledge of the message told Reuters last week.

US Ambassador Laura Kennedy said she found North Korea's threat on Tuesday profoundly disturbing and later tweeted that it was "offensive".

Poland's representative suggested North Korea's participation in the UN forum should be limited.

Impoverished and malnourished North Korea is one of the most heavily sanctioned states in the world.

It is still technically at war with South Korea after a 1950-53 civil war ended in a mere truce.

Washington and its allies are believed to be pushing to tighten the noose around North Korea's financial transactions in a bid to starve its leadership of funding.

Jon said last week's test was an act of self-defence against nuclear blackmail by the United States, which wanted to block North Korea's economic development and its fundamental rights.

"It is the disposition and firm will of the army and people of the DPRK to counter high-handed policy with tough-fist policy and to react to pressure and sanctions with an all-out counter-action," he said.

Jon said the United States had conducted most of the nuclear tests and satellite launches in history, and he described its pursuit of UN Security Council resolutions against North Korea as "a breach of international law and the height of double standards".

Neither Russia nor China, which are veto-wielding members of the UN Security Council, spoke at Tuesday's meeting in Geneva.

Before its nuclear test, North Korea was already facing growing diplomatic pressure at the United Nations.

The UN Human Rights Council is widely expected to order an inquiry next month into its leaders' responsibilities for crimes against humanity.

- Reuters