The latest harsh rhetoric from the isolated regime appeared aimed at heightening tensions on the divided peninsula and could be a test for Barack Obama days before he is sworn in as the new U.S. president.
The North's Korean People's Army called South Korean President Lee Myung-bak a "traitor" and accused him of preparing a military provocation, according to a statement carried Saturday by the North's state-run Korean Central News Agency.
Pyongyang said it was adopting "an all-out confrontational posture" and warned of a "strong military retaliatory step." South Korea immediately put its forces on alert.
Seoul's Yonhap news agency reported Sunday that the South has significantly beefed up forces along its heavily armed land border with the North and near their disputed western sea border. But the presidential office and the Defense Ministry denied the report.
A Defense Ministry official said Sunday that the South's military will remain on alert, though there were no unusual moves by the North's forces. The official spoke on condition of anonymity citing department policy.
The North has issued similar threats in the past in anger over the hard-line policies Lee has implemented since taking office last year. Lee ended previous administrations' unconditional aid to North Korea, but has also called for dialogue.
Saturday's threat from Pyongyang appeared more serious, however, because a uniformed military officer — flanked by military unit flags — read the statement instead of the usual television newsreader.
Analysts said the North's latest saber rattling appears to be a negotiating tactic aimed at Seoul and Washington ahead of Obama's inauguration Tuesday in the U.S.
Kim said Pyongyang is trying to use raised tensions to make a case for its long-standing demand for a peace treaty and establishment of diplomatic ties with Washington — the regime's top foreign policy goal.
South Korea, the U.S. and three other nations have sought to coax North Korea — which detonated an atomic device in 2006 — to give up its nuclear program by offering aid for disarmament. The pact has been deadlocked over how to verify North Korea's past nuclear activities.
Despite the impasse, Seoul's deputy nuclear negotiator has been visiting the North since Thursday. The trip — the highest-level visit to the North in a year — was seen as an indication Pyongyang has not abandoned the disarmament pact.
Nuclear envoy Hwang Joon-kook and his team visited the North's main nuclear complex at Yongbyon on Friday. They will hold talks through Monday before returning home, said presidential spokeswoman Kim Eun-hye.
The two Koreas have been separated one of the world's most heavily armed borders since a three-year war ended in a truce in 1953.
Ties warmed significantly following the first-ever summit of their leaders in 2000, but the reconciliation process came to a halt after Seoul's conservative, pro-U.S. leader Lee came to power last year.