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

President Lee Myung-bak travels to Washington on Monday for talks with President Barack Obama that are 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 stops 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 a long-suspected uranium enrichment program for the first time. Both plutonium and uranium are key ingredients of atomic b****s.

A commentary published Saturday in the North's state-run Tongil Sinbo weekly claimed the U.S. 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," it said.

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

President Lee summoned his top security ministers Sunday 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's crucial that the U.S. and other nations "make sure those sanctions stick."

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

"God only knows what he wants," Biden said of Kim. "There's all kinds of discussions. 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 and threat to the world, and particularly East Asia," the vice president said.

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

"Kim Jong Un's status is still unstable. Kim Jong Il appears to be trying to give the son a powerful means to strengthen his succession," Lee said. "Kim Jong Un could also get the credit for nuclear weapons development."

North Korea is already believed to have enough plutonium for at least half a dozen atomic b****s.

North Korea says its nuclear program is a deterrent against the U.S., which it accuses of plotting to invade and topple its regime. Washington, which has 28,500 troops in South Korea, has repeatedly denied having any such plans.