Pyongyang insists its launch carried a communications satellite
North Korea has started to reprocess spent fuel rods at its nuclear plant, says the country's state media.
The reprocessing is a possible move towards producing weapons grade plutonium and comes after Pyongyang's launch of a long-range rocket in April.
Earlier, the UN Security Council imposed sanctions on three North Korean companies in response to the launch.
Pyongyang said it would ignore the sanctions, describing them as "a wanton violation of the UN charter".
"The reprocessing of spent fuel rods from the pilot atomic power plant began as declared in the Foreign Ministry statement dated 14 April," North Korea's Central News Agency (KCNA) quoted a Foreign Ministry official as saying.
That was the date that Pyongyang announced it was pulling out of talks on its nuclear programme and would restart its Yongbyon reactor.
The official said the reprocessing would "contribute to bolstering the nuclear deterrence for self-defence in every way to cope with the increasing military threats from the hostile forces".
The BBC's John Sudworth in Seoul says this announcement is confirmation that - for now at least - North Korea is serious about turning its back on the six-party talks, the long-running nuclear disarmament negotiations involving its four closest neighbours and the United States.
North Korea had already partially dismantled its nuclear reactor - the source of material for a 2006 atomic test.
But it now says it is reprocessing remaining spent fuel rods, which experts say could provide material for at least one more nuclear bomb.
The country is already thought to possess enough reprocessed plutonium for between six and eight nuclear weapons - so in the immediate term the announcement does not significantly alter the strategic balance, our correspondent says.
- North Korea agrees to close its main nuclear reactor in exchange for fuel aid
- North Korea shuts its main Yongbyon reactor
- North Korea makes its long-awaited declaration of nuclear assets
- The US removes North Korea from its list of countries which sponsor terrorism
- Pyongyang slows work to dismantle its nuclear programme after a US decision to suspend energy aid
- The North says it is scrapping all military and political deals with the South, accusing it of "hostile intent"
5 April 2009
- Pyongyang launches a rocket carrying what it says is a communications satellite
14 April 2009
- After criticism of the launch from the UN Security Council, North Korea vows to walk out of six-party talks
But there will be deep concern in Washington and Seoul that the nuclear talks process, which began in 2003, seems to be unravelling fast and a defiant North Korea is arming itself once again, he adds.
Pyongyang's announcement came only hours after the UN imposed sanctions on three companies it said had supported North Korea's controversial rocket launch, as well as updating the list of goods and technologies already banned.
The North says the launch put a satellite in orbit broadcasting patriotic songs, but critics say it was a long-range missile test that crashed into the Pacific.
The sanctions mark the first concrete steps against Pyongyang since the UN officially condemned the launch.
The UN's Sanctions Committee head said the new sanctions aimed partly at denying North Korea access to some of the latest technologies relevant to ballistic missile programmes.
North Korea's Deputy UN Ambassador Pak Tok Hun said the decision was "a wanton violation of the United Nations charter".
"It is the inalienable right of every nation and country to make peaceful use of outer space," he said.
"That is why we totally reject and do not recognise any sort of decision which has been made in the Security Council."
The debate over sanctions has been complicated by opposition from Russia and China, a major trading partner, over how to respond to North Korea's actions.
Russian Foreign Minister Sergei Lavrov, who has been in Pyongyang in an attempt to persuade the North to return to the nuclear talks, said earlier that sanctions were "not constructive".
But US diplomats said the new sanctions were "a serious and credible response" to the launch.
The UN Security Council council unanimously condemned North Korea's rocket launch on 13 April, saying it was a cover for a long-range missile test and as such contravened a 2006 resolution banning such tests.