"We've taken serious steps of boosting our nuclear arsenal and we are also prepared to mobilise all of our military force against any provocative moves by the enemy," Yonhap quoted the North's state KCNA news agency as saying on Monday.
It was the first time Pyongyang said it had boosted its nuclear weapons programmes - it had said last week it may increase its arsenal to maintain a balance of power in East Asia and help prevent a US attack.
During a tour of Asia last week, US Secretary of State Condoleezza Rice told North Korea it must immediately return to talks on its nuclear programme.
Rice, on her first tour of Asia since taking office, said North Korea could only realise its wishes for security assurances if it ended its boycott of six-nation talks on its nuclear ambitions.
Rice called North Korea an
'outpost of tyranny'
North Korea last took part in the talks in June 2004 and in February announced a new suspension of dialogue, demanding Rice apologise for calling the communist state an "outpost of tyranny".
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 it.
Proliferation experts said the country may have one or two nuclear weapons, and could possibly have eight or more.
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.
A fourth round planned for late 2004 never materialised.
In a recent interview, 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.