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South Korea warned North Korea (search) on Monday against conducting a nuclear test, saying one would further isolate the communist state and undermine its security. The United States called the North's resistance to international disarmament talks unacceptable.

Concerns that the isolated North is trying to develop a nuclear arsenal have escalated after it apparently shut down a nuclear reactor recently — a move that could allow it to harvest weapons-grade plutonium.

South Korea's foreign minister, Ban Ki-moon, said in a speech on Monday that North Korea "cannot have its future guaranteed" if it conducts a nuclear test.

"Nuclear weapons can never guarantee North Korea's security and will only bring about and worsen the isolation of its politics and economy," Ban said, according to South Korea's Yonhap news agency.

The South Korean warning came after U.S. media reported over the weekend that Pyongyang (search) might be preparing for its first nuclear test and North Korea threatened to bolster its "nuclear deterrent."

North Korea, meanwhile, lashed out at Secretary of State Condoleezza Rice for recently saying that Washington was willing to take the nuclear issue to the United Nations.

"If the United States wants so much to drag the nuclear issue to the U.N. Security Council, it may do so," North Korea's Foreign Ministry spokesman said, according to the North's official Korean Central News Agency. "However, we want to make clear that we will regard sanctions as a declaration of war."

North Korea declared in February that it had nuclear weapons and was boycotting international disarmament talks, which also involve the United States, China, South Korea and Russia. Since then, efforts to get the North back to the bargaining table have floundered.

In the latest diplomatic push, Washington's top envoy on the North Korean nuclear issue met with South Korean officials Monday and discussed ways to revive the negotiations.

"What we are focusing on is the diplomatic track and the need to get the talks going, and more importantly, once they get going, to achieve progress in the talks," Christopher Hill, the U.S. assistant secretary of state for East Asian and Pacific Affairs, said following his meeting with his South Korean counterpart, Song Min-soon.

Washington, however, is reportedly exploring other options in stopping North Korea from building up its alleged nuclear arsenal.

The New York Times reported in its Monday editions that the Bush administration is debating a plan to seek a U.N. resolution allowing countries to intercept shipments in or out of North Korea that may contain nuclear materials or components.

The proposed resolution, promoted by a growing number of senior administration officials, would enable the U.S. and other nations to intercept shipments in international waters off the Korean Peninsula, and force down aircraft for inspection, the Times reported.

The United States has told China and its other negotiating partners that it has serious concerns about "recent provocative statements" by North Korea on its nuclear weapons intentions.

During three previous rounds of negotiations, North Korea has claimed to have nuclear capability and the potential to demonstrate it.

American analysts have said during the past week that they believe some of the claims are genuine. U.S. intelligence analysts have estimated in the past that North Korea has produced at least two nuclear bombs.