South Korea's defense minister says his country will beef up its military capabilities, now that North Korea appears to have tested a nuclear weapon. North Korea also threatens tough action to resist pressure from the United States.

South Korean Defense Minister Yoon Kwang-ung told lawmakers in Seoul the country will likely boost its ability to respond to a potential nuclear threat from North Korea.

He says if North Korea has nuclear capabilities, the South will "improve and enlarge" its conventional forces, but will not develop nuclear weapons.

South Korea in 1991 signed an agreement with North Korea to keep the peninsula free of nuclear arms. Seoul says Pyongyang violated that with its announcement of a nuclear weapons test. Experts are still trying to confirm whether Monday's underground blast in North Korea was a nuclear device.

South Korea has about 650,000 active duty troops, while the United States deploys about 28,000 soldiers here to deter North Korea from repeating its 1950 invasion of the South.

General Burwell Bell, commander of U.S. Forces in South Korea, issued a statement to his forces Wednesday assuring them the alliance can deter, and defeat, any North Korean attack. He says U.S. forces have been well trained to confront nuclear, biological, and chemical threats.

Pyongyang says it needs nuclear arms to deter aggression by the United States. Washington says it has no intention of attacking the North, and has joined regional partners in offering economic and diplomatic benefits if Pyongyang gives up its nuclear weapons programs.

Now, the United States is working with the U.N. Security Council to pass a resolution punishing North Korea with economic sanctions.

Meanwhile, the North Korean Central News Agency warned it would view such sanctions as a declaration of war.

North Korea said it would respond with "physical measures" to pressure from the United States, but did not say what they would be. It also said it is prepared for "both dialogue and confrontation" to resolve the crisis over its nuclear programs.

Japan is taking its own steps to punish the North. Japanese news media reported Wednesday that all North Korean ships would be barred from Japanese ports, effectively cutting off trade between the two countries and the flow of cash from ethnic Koreans in Japan to the impoverished communist country.

North Korea's nuclear test has opened an intense debate in South Korea over what went wrong - and how to fix it. On Wednesday, sharp rhetoric prevented lawmakers from agreeing on language for a resolution condemning Pyongyang's move.

Opposition lawmakers blame President Roh Moo-hyun for being too indulgent toward North Korea. Seoul has poured billions of dollars into the impoverished North, hoping to ensure its neighbor would remain peaceful and nuclear-free.

The original architect of South Korea's "sunshine" policy of engagement with Pyongyang, former South Korean President Kim Dae-Jung, told university students Wednesday there was still hope for resolving the crisis.

He says North Korea must dismantle its nuclear weapons - and calls on the United States to hold bilateral talks with Pyongyang.

Washington insists any dialogue with Pyongyang must happen within multinational talks aimed at denuclearizing the North. Those talks also involve Japan, China, Russia, and South Korea.

There are signs South Korea is shifting to a tougher policy toward the North. President Roh has said the engagement policy would need to change following the nuclear test. And in at least one South Korean newspaper poll, a majority of respondents said South Korea should acquire its own nuclear weapons to counter Pyongyang.

Early Wednesday, Japanese news media reported signs of a second North Korean nuclear test. Officials in the U.S., South Korea and Japan, however, said there were no indications of a new test.