North Korea said Thursday it would strengthen its nuclear deterrent and other defences regardless of cost, amid fears it is preparing for another atomic weapons test.

"No matter what high expense the army and people of the DPRK (North Korea) have to cover, they will further bolster its defence capability including nuclear deterrence to firmly protect its dignity and sovereignty," said a state body known as the Committee for the Peaceful Reunification of Korea.

The committee, in comments on the official news agency, vowed not to be deceived by offers from hostile forces that it could get benefits in return for abandoning nuclear weapons and suspending missile launches.

It was the second such pledge in five days.

On Sunday a foreign ministry spokesman condemned a statement by the five permanent UN Security Council members which urged Pyongyang to refrain from nuclear tests or other actions that could spark security concerns.

The North would safeguard the nation "based on self-defensive nuclear deterrent" and strongly pursue peaceful space development, the spokesman said.

There has been widespread speculation the North could stage its third nuclear test following a failed rocket launch on April 13 that sparked international criticism.

Satellite photos have shown work in progress at the Punggye-ri nuclear test site.

The North said its launch was intended to put a peaceful satellite into orbit. The United States and its allies said the exercise was a pretext for a ballistic missile test banned under UN resolutions.