North Korea says it has resumed reprocessing fuel rods at Yongbyon
North Korea has threatened to carry out nuclear missile tests unless the UN Security Council apologises for its condemnation of a recent rocket launch.
Pyongyang said it would be compelled to take self-defence measures "including nuclear and intercontinental ballistic missile tests" if no apology was made.
When North Korea launched its rocket on 5 April, the launch was seen by the US and others as a disguised missile test.
The UN denounced North Korea's actions and called for tighter sanctions.
Pyongyang has insisted it put a communications satellite in orbit, and said it would ignore the sanctions, describing them as "a wanton violation of the UN charter".
North Korea conducted its first and only nuclear test in October 2006.
The BBC's John Sudworth in Seoul says preparations for a second atomic test would cause serious international concern.
He says it would also confirm that North Korea is no longer taking part in the long-running international nuclear disarmament negotiations known as the six-party talks.
North Korea's foreign ministry said the UN should apologise for "infringing" the country's sovereignty and retract "all its resolutions and decisions" against Pyongyang.
It also announced plans to build a light-water nuclear reactor, according to the statement carried by the official Korean Central News Agency.
Following the UN criticism, the North pulled out of international negotiations on its denuclearisation and ejected all monitors from the country.
Pyongyang has since announced that it has started reprocessing spent fuel rods at its Yongbyon nuclear plant.
The reprocessing is a possible move towards producing weapons-grade plutonium.
North Korea had partially dismantled its nuclear reactor under a deal agreed at international talks in early 2007, in which it was also promised fuel aid.
It is thought to possess enough reprocessed plutonium for between six and eight nuclear weapons.
However, analysts say Pyongyang has not yet mastered the technology to make a nuclear warhead small enough to place on a missile.