North Korea said it is bolstering its nuclear capability to cope with what it sees as hostile US policy and military threats amid tensions over the sinking of a South Korean warship.
The North's military also accused the US and South Korea of bringing heavy weapons to the Korean border village of Panmumjom and vowed to take "strong military countermeasures" if they are not withdrawn.
An international investigation concluded last month that North Korea torpedoed the warship Cheonan near the tense Korean sea border. North Korea flatly denies the allegation and has warned any punishment would trigger war. Forty-six South Korean sailors died in the sinking.
"The recent disturbing development on the Korean peninsula underscores the need for (North Korea) to bolster its nuclear deterrent in a newly developed way to cope with the US hostile policy," the North's foreign ministry said.
The statement, carried by the official Korean Central News Agency, did not elaborate how North Korea would strengthen its nuclear capability. But analysts said it was threatening to manufacture bombs based on newer processes, such as uranium-enrichment or nuclear fusion, as international criticism of North Korea mounts over the sinking.
North Korea said last year it was in the final stages of enriching uranium, a process that could give it an easier, second way to make nuclear bombs, in addition to its existing plutonium-based programme. In May, North Korea claimed its scientists succeeded in creating a nuclear fusion reaction - technology necessary to manufacture a hydrogen bomb.
North Korea cites the threat of a nuclear attack from the US as the main reason behind its drive to build atomic weapons, although the US has repeatedly said it has no intention of attacking.
The North's warning came after world leaders at the G8 Summit criticised its nuclear programme. The leaders also condemned the attack that led to the sinking of the Cheonan, citing an independent report that found North Korea was responsible.
The US military said it was checking the North Korean statement.
The 1950-53 Korean War ended in an armistice rather than a peace treaty. The United States stations about 28,500 troops in South Korea as a deterrent against the North.