SEOUL - North Korea Tuesday accused the United States of stepping up production and deployment of "bunker-buster" bombs targeting its nuclear sites.

The United States is deploying the bombs "to attack underground military targets and nuclear facilities" in the North, the ruling communist party newspaper Rodong Sinmun said in a commentary.

This proved that Washington has not abandoned "ambitions to stifle" Pyongyang by force, it said.

Government newspaper Minju Joson carried a similar commentary, saying the United States was producing such bombs for a pre-emptive strike on the underground facilities.

"The only choice our republic can make at a time when its dignity and safety is under threat is to strengthen its war deterrence by all means," it said.

The US military used laser-guided bunker-buster bombs during the 1990-91 Gulf War to destroy underground command centres in Iraq.

The South's unification ministry said in a report last week that North Korea has 20 nuclear-related sites manned by an estimated 3,000 workers.

Eleven of the facilities are at the Yongbyon nuclear complex and there are also nine uranium-related mines and other facilities, it said.

The North shut down Yongbyon in 2007 under a six-nation nuclear disarmament deal. In April it quit the forum and announced it was resuming the reprocessing of plutonium from spent fuel rods at the reactor there.

It carried out its second atomic weapons test in May and is thought to have enough plutonium to make maybe six to eight such weapons.

The North also says it is in the final phase of an experimental highly enriched uranium programme - another way to make an atomic bomb.

The North has expressed willingness to return to the six-party forum but only if it first holds satisfactory talks with Washington.

Envoys from the two sides held rare face-to-face talks in New York Saturday in an apparent preparation for a possible bilateral meeting.