Officials here admitted they could not rule out the possibility that the North would conduct an underground nuclear bomb test to grab the attention of Washington, whose focus was primarily on Iran's nuclear program and the Middle East crisis.

"At this moment, we cannot rule out the possibility (of the North's nuclear test)," said Foreign Minister Ban Ki-moon, speaking to journalists on Wednesday.

South Korea is closely monitoring North Korea's nuclear activities around the clock and has dispatched military personnel to a state-run seismology institute to monitor for possible underground explosions, according to Ban and defense officials.

"The South Korea government is cooperating with relevant nations and is closely monitoring North Korea's activities through various channels," Ban said. "We are trying to gather as much information as we can."

The diplomatic chief also warned of serious consequences if the North conducts a nuclear weapons test, defying international calls. "(A nuclear test) would bring about much more serious consequences than its missile test last month," Ban said.

"It would pose a serious threat that would shake the international non-proliferation system from its foundation, and North Korea would be further isolated," he said.

U.S. television network ABC said last week said that North Korea may be preparing an underground nuclear test. A U.S. intelligence agency had recently detected "suspicious vehicle movement" at a suspected nuclear test site in North Korea, a senior military official was quoted as saying by ABC News.

North Korea claimed in Feb. 2005 that it possessed nuclear weapons, declaring itself a nuclear power. The communist state also boasted of having extracted more weapons-grade plutonium to make additional atomic weapons.

Ratcheting up tensions, North Korea test-fired a set of missiles on July 5, including a long-range ballistic missile. Washington has been put alert as the North's missile, which could be equipped with a nuclear warhead, may be capable of reaching the continental United States.

In response, the U.N. Security Council unanimously passed a strong resolution on July 15 condemning North Korea for its multiple missile launches and barring the country from acquiring or selling missile technology and materials related to weapons of mass destruction.

In protest against the U.N. measures, Pyongyang vowed to take "stronger physical action," indicating a nuclear weapons test.

Seoul's fear was heightened this week as the North strongly responded to an annual South Korea-U.S. joint military exercise.

The South Korean military said the computer war games are purely defensive, but North Korea denounced them as a preparation for an attack, saying it would no longer be bound to an armistice treaty that ended the 1950-53 Korean War.

The exercise is "an undisguised military threat and blackmail against the DPRK (North Korea) and a war action," the North's military said in its Tuesday statement.

"The (North) Korean Peoples' Army side, therefore, reserves the right to undertake a pre-emptive action for self-defense against the enemy at a crucial time it deems necessary to defend itself," it said.

The two Koreas are still technically in a state of war since the Korean War ended with an armistice, not a peace treaty.

For its part, the United States has stepped up pressure on North Korea to give up its missile and nuclear weapons program.

The U.S. Treasury Department has forced foreign banks to freeze North Korean accounts, further squeezing the cash-strapped Kim Jong Il regime. Pyongyang has already walked out of the multilateral six-party talks on its nuclear drive in protest against Washington's financial sanctions imposed last September.

U.S. President George W. Bush also asked China's President Hu Jintao to put pressure on Kim Jong Il to abandon his country's nuclear ambitions.

Washington and Beijing need "to continue to work together to send a clear message to the North Korean leader that there is a better choice for him than to continue to develop a nuclear weapon," Bush said.

Some analysts in Seoul call for South Korean President Roh Moo-hyun to seek summit talks with the North Korean leader to seek ways to resolve the security crisis on the peninsula.

"An inter-Korean summit could provide a fresh momentum to resolve the nuclear and missile issues at a time when dialogue between Pyongyang and Washington remains cut off," said Paik Hak-soon, a North Korea expert at South Korea's private Sejong Institute.

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