A North Korean Foreign Ministry statement carried by the official KCNA news agency on Thursday said the country would seek to strengthen its nuclear arsenal and accused the US of plotting to overthrow the government.
"We had already taken the resolute action of pulling out of the [Nuclear Non-proliferation Treaty] and have manufactured nukes for self-defence to cope with the Bush administration's evermore undisguised policy to isolate and stifle the Democratic Peoples Republic of Korea (DPRK)," the statement said.
The statement added that Pyongyang would also suspend its involvement in six-nation talks.
"The US disclosed its attempt to topple the political system in DPRK at any cost, threatening it with a nuclear stick"
North Korean Foreign Ministry
"We have wanted the six-party talks but we are compelled to suspend our participation in the talks for an indefinite period," it said.
The six nations taking part in the talks are North Korea, China, Japan, Russia, South Korea and the United States.
"The US disclosed its attempt to topple the political system in DPRK at any cost, threatening it with a nuclear stick.
"This compels us to take a measure to bolster our nuclear weapons arsenal in order to protect the ideology, system, freedom and democracy chosen by the people in the DPRK."
The Foreign Ministry statement was Pyongyang's first official reaction to US President George Bush's State of the Union address last week.
In his key policy address before Congress, Bush avoided inflammatory language aimed directly at the Stalinist regime, saying only that Washington was working closely with its allies.
North Korea admits to having
Three years ago he named the communist country part of an "axis of evil" along with Iran and Iraq.
Pyongyang's statement expressed anger at being branded an "outpost of tyranny" by US Secretary of State Condoleezza Rice, citing that as proof of Washington's "hostile policy" towards the Asian nation.
"The Bush administration termed the DPRK - its dialogue partner - an 'outpost of tyranny', putting into the shade its hostile policy, and totally rejected it. This deprived the DPRK of any justification to participate in the six-party talks," it said.
The nuclear stand-off erupted in October 2002 when the US accused North Korea of operating a programme based on highly enriched uranium, violating a 1994 arms control agreement.
Pyongyang denied that charge but restarted a plutonium programme.
"The absence of progress in six-party talks means they are making further progress towards their increased capability [in weapons]"
John Bolton, US undersecretary of state for arms control
North Korea attended three rounds of the six-nation talks.
But it shunned a fourth round set for last September, complaining of "hostile" US policies.
The North's statement came as the US and its allies stepped up diplomacy to re-launch the six-nation talks.
South Korean Foreign Minister Ban Ki-Moon left for Washington on Thursday for talks with Rice and other US officials.
In Tokyo, John Bolton, US undersecretary of state for arms control and international security, accused North Korea of continuing its weapons programme.
"The absence of progress in six-party talks means they are making further progress towards their increased capability [in weapons]", he said.
But he said Washington had not imposed a deadline for Pyongyang to return to the table.
"There is no deadline here," he said. "To be productive, you have to have talks and that's why we are waiting."