North Korea Monday vowed again to bolster its nuclear deterrent, saying the current US administration is "nothing different" from its predecessor.

North Korea is "firmly convinced that it was entirely just when it opted for bolstering the nuclear deterrent to protect the sovereignty", a foreign ministry spokesman said in a statement published by state media.

The North last week threatened to conduct a second nuclear test and ballistic missile tests unless the United Nations apologises for condemning and punishing its April 5 rocket launch.

Pyongyang said it put a peaceful satellite into orbit but the United States, South Korea and Japan said it staged a disguised missile test.

The satellite, which was "accurately" put into orbit, is "regularly going round the earth", the foreign ministry spokesman said.

"It is only the US administration and unsavory forces subservient to it that insist (North Korea's) satellite launch was a ballistic missile launch," he said.

He denounced Washington for trying to find an excuse for applying sanctions against Pyongyang "at any cost" and to suffocate its defence industry.

The US government is "nothing different from the preceding administration which frantically worked to stifle by force other countries which incurred its displeasure", he said.

The UN Security Council on April 13 slapped sanctions on North Korea, banning transactions and calling on UN member-states to freeze the assets of three business entities of Pyongyang.

In protest, the North said it had started reprocessing spent fuel rods at its Yongbyon complex to make weapons-grade plutonium.

The North had already responded to the UN criticism by announcing it was quitting a six-nation nuclear disarmament pact with the United States, China, South Korea, Japan and Russia.

Analysts say the North's threat to conduct a second nuclear test, following its first in October 2006, is aimed at forcing the US to open direct negotiations.