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 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.

The criticism came despite the North's attempts to set up a meeting with the United States, as it comes under sanctions pressure over its nuclear ambitions.

The North in April abandoned a six-nation nuclear disarmament deal and announced it was resuming the reprocessing of plutonium at its Yongbyon complex.

It carried out its second atomic weapons test in May, triggering tougher United Nations sanctions.

This month the North 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.

South Korea has reacted cautiously to recent peace overtures from its neighbour.

Vice Unification Minister Hong Yang-Ho told a forum Tuesday there are no signs so far it is willing to give up its nuclear weapons.

Hong said Pyongyang has shown "no substantive change" to its position on nuclear disarmament.

In an unusual development, the North's state media Tuesday reported that a South Korean pig farmer has defected across the heavily-guarded border and is now "under the warm care" of authorities.

The South-North defection, if confirmed, would be extremely rare. Seoul officials said they were checking the report.

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