South Koreans watch a television broadcasting North Korea's nuclear test at a railway station in Seoul, Monday, October 9, 2006

The leaders of South Korea and Japan are joining a chorus of condemnation around the world following North Korea's apparent entry into the nuclear weapons club. Pyongyang says it has conducted its first nuclear test, and international leaders are warning there will be consequences.

South Korean President Roh Moo-hyun says his country's response to an apparent nuclear test by communist North Korea will be "stern but fair."

Early Monday, seismographs around the world detected a tremor in North Korea that may have been caused by an underground nuclear blast. Within hours, a North Korean broadcaster brought a triumphant announcement from Pyongyang.

The announcer says the test produced no radiation leakage, because it was conducted with wisdom and scientific knowledge. She adds the historic event brings the army and the North Korean people great pleasure.

North Korea says it conducted the test to improve the country's defenses.

Pyongyang has said it needs nuclear weapons to deter an attack by the United States. The United States says it has no intention of attacking North Korea, and has called for it to comply with past pledges to remain nuclear-free.

South Korean authorities have responded by halting emergency flood aid destined for the impoverished North. In addition, Seoul raised the alert status for its military forces.

After meeting with visiting Japanese Prime Minister Shinzo Abe, President Roh said the apparent nuclear test will "seriously affect" South Korean policy.

Describing it as both "a warning and a prediction," Mr. Roh says it is now very difficult for Seoul to pursue or justify its policy of engagement with North Korea.

South Korea has poured billions of dollars in aid into the North over the past six years - with the goal of improving relations with its former foe.

Prime Minister Abe said the test is a threat to North Asian security and stability. He says he will push hard for an international response.

Mr. Abe says we are now living in a new and dangerous nuclear era. His government will study taking stern measures against North Korea in coordination with concerned countries and in cooperation with the U.N. Security Council.

The U.N. Security Council is expected to take up the matter in New York within a few hours.

The U.S. government described the North's apparent test as a "provocative act," that defies international will.

North Korea promised South Korea, Japan, the United States, China and Russia last September it would give up its nuclear weapons in exchange for economic and diplomatic benefits. South Korea chastised the North for negating that promise and damaging the six-party nuclear disarmament process.

China, North Korea's closest ally, condemned the North Korean action, as did Russia.

If confirmed, the test will make North Korea the eighth declared nuclear power. The United States, China, Russia, Britain, France, India and Pakistan are the other nations known to have nuclear weapons. Israel is thought to have them, but has never confirmed that it does.