Pyongyang's latest statement on Tuesday comes as US Secretary of State Condoleezza Rice leaves for Asia to push for a restart of stalled six-party talks aimed at curbing North Korea's nuclear ambitions.
US President George Bush's administration has repeatedly said the United States has no intention of invading North Korea.
"The DPRK (Democratic People's Republic of Korea) will take necessary counter-measures, including bolstering of its nuclear arsenal to cope with the extremely hostile attempt of the US to bring down the system in the DPRK," the North's official KCNA news agency quoted a foreign ministry spokesman as saying.
"The reality testifies to the fact that the DPRK's nuclear weapons serve as a powerful deterrent to keep the equilibrium of forces in the region, avert a new war and ensure peace," the spokesman said.
"The reality testifies to the fact that the DPRK's nuclear weapons serve as a powerful deterrent to keep the equilibrium of forces in the region, avert a new war and ensure peace"
North Korea Foreign Ministry
The comments came at the end of a long critique of regular US-South Korean joint military training exercises, which the North condemns each time they are held.
On 10 February, North Korea officially declared for the first time that it had nuclear weapons. It said it needed the weapons to counter what it saw as Washington's hostile policies towards the reclusive state.
Proliferation experts said the country may have one or two nuclear weapons, and could possibly have eight or more.
When Pyongyang made its nuclear revelation over a month ago, it also said it was pulling out of disarmament talks. It later hinted at a return to the table if the conditions were right and the US showed what it called sincerity.
Since August 2003 there have been three inconclusive rounds of talks on dismantling North Korea's weapons programmes involving the two Koreas, the US, China, Japan and Russia.
Rice will discuss the nuclear issue
with other Asian officials
A fourth round planned for late 2004 never materialised.
In an interview last Friday, Rice said Pyongyang was throwing up smoke screens in its continued refusal to return to the six-way talks.
"Let's be realistic here. This isn't an issue of what we say or what we don't say. This is an issue of whether the North Koreans come to the table prepared to make a strategic choice" to give up their nuclear activities, she said.
Rice said she would discuss with Chinese, South Korean and Japanese officials "what other steps need to be taken" to deal with the nuclear issue but declined to provide details.
There has been a flurry of diplomatic activity in the weeks leading up to Rice's visit to Asia, as negotiators and government officials have shuttled around for meetings as they try to coax Pyongyang back to the negotiating table.
South Korean defence chief Yoon Kwang-ung will visit China later this month to talk about the North's nuclear weapons programmes with officials in Beijing, South Korea's Yonhap news agency reported.