SEOUL, South Korea - South Korea's president ordered his top security officials yesterday to deal "resolutely and squarely" with new North Korean warnings of a nuclear war on the eve of his visit to Washington, where Vice President Biden said, "God only knows" what North Korea wants from the latest showdown.

President Lee Myung-bak travels to the United States today for talks with President Obama that were expected to focus on the North's rogue nuclear and missile programs.

The trip comes after North Korea's Foreign Ministry threatened war with any country that stopped its ships on the high seas under new sanctions approved by the U.N. Security Council in response to its May 25 nuclear test.

It also vowed Saturday to "weaponize" all its plutonium and acknowledged, for the first time, a long-suspected uranium-enrichment program. Both are key ingredients of atomic bombs.

A commentary published Saturday in the North's state-run Tongil Sinbo weekly contended that the United States was deploying a vast number of nuclear weapons in South Korea and Japan.

North Korea "is completely within the range of U.S. nuclear attack, and the Korean peninsula is becoming an area where the chances of a nuclear war are the highest in the world," the commentary said.

Kim Yong-kyu, a spokesman at the U.S. military command in Seoul, denied the allegation, saying the United States no longer has nuclear bombs in South Korea. U.S. tactical nuclear weapons were removed in 1991 as part of arms reductions after the Cold War.

President Lee summoned his top security ministers yesterday and ordered them to "resolutely and squarely cope" with the North's threats, his office said. The Unification Ministry, responsible for ties with the North, issued a statement demanding that it stop inflaming tension and resume talks with the South.

"North Korea should give up its nuclear program . . . and stop any kind of military threat," it said. "We urge North Korea to respond in a sincere dialogue to improve South-North Korean relations."

The new U.N. sanctions approved Friday are aimed at depriving the North of the financing used to build its nuclear program. They also authorize searches of North Korean ships suspected of transporting illicit ballistic missile and nuclear materials.

Biden told NBC's Meet the Press that it was crucial that the United States and other nations "make sure those sanctions stick."

North Korea's leader, Kim Jong Il, reportedly had a stroke 10 months ago, and analysts say they believe there may be a plan to name his inexperienced 26-year-old son, Kim Jong Un, as his successor.

"God only knows what he wants," Biden said of Kim. "Whether this is about succession, wanting his son to succeed him. Whether or not he's looking for respect. Whether or not he really wants a nuclear capability to threaten the region. . . . We can't guess his motives.

"We just have to deal with the reality that a North Korea that is either proliferating weapons and or missiles, or a North Korea that is using those weapons . . . is a serious danger," Biden said.

Lee Sang-hyun, an analyst at the Sejong Institute, a South Korean security think tank, said he believed the North would continue to conduct nuclear tests until it mastered the technology to mount nuclear warheads on missiles and would give credit for it to Kim Jong Un.

"Kim Jong Un's status is still unstable," Lee said. "Kim Jong Il appears to be trying to give the son a powerful means to strengthen his succession."