His comments were the US government's toughest yet since North Korea announced that it intends to conduct its first nuclear test to bolster its deterrent against US threats and sanctions.

But experts consulted here said the United States has no viable military options, and if it did strike North Korea it would invite potentially devastating retaliation against Japan or South Korea.

"What would we attack?" said Robert Einhorn, a former assistant secretary of state for non-proliferation.

Einhorn said the United States suspects North Korea has a uranium enrichment program and enough plutonium for 10 or 11 weapons. But, he said, "We don't have a clue where it is."

"What would we be gaining with a military strike?"

US Defense Secretary Donald Rumsfeld and Pentagon spokesmen have refused to comment on what preparations the US military is taking in anticipation of a North Korean nuclear test.

Japanese news reports said a US WC-135 aircraft equipped to gather and test air samples for signs of a nuclear blast took off from Kadena Air Base in Okinawa on a monitoring mission off North Korea. US military spokesmen would not comment on the report.

The United States has submarines and warships armed with cruise missiles in the Pacific and long-range B-52 bombers in Guam that could be used if President George W. Bush ordered air strikes.

Analysts acknowledge that air strikes are not entirely outside the realm of possibility.

Former defense secretary William Perry and a former top Pentagon strategist, Ashton Carter, called in June for cruise missile strike to stop North Korea from testing a long-range Taepodong-2 missile.

Their idea was ignored and the North Korean missile failed shortly after launch on July 4-5.

But it was the first time that military action was seriously raised as an option after years of fruitless diplomatic efforts to get North Korea to give up its nuclear program.

"This is a far more threatening development than the missile test and so the balance will tilt somewhat in the direction of that argument," said Michael Levi, an expert on the Council on Foreign Relations, in an online question and answer session hosted by the CFR.

"Now, where it balances out is difficult to tell because there are still immense downsides and dangers to any sort of strike," said Levi.

North Korea could inflict massive casualties in Seoul with its 11,000 artillery pieces and large stockpile chemical rounds, experts say.

Although North Korea is not known to have armed missiles with nuclear warheads, some analysts say it should not be ruled out.

John Pike, director of Global Security.Org, cites nuclear cooperation between the North Koreans and Pakistanis, and speculates that Pakistan may have tested a North Korean warhead in one of two nuclear tests conducted in 1998.

"North Korea may well have nuclear warheads on top of missiles today," said Bruce Bennett, a North Korea expert at Rand Corporation, a Washington think tank.

"And if we go and start attacking like their nuclear facilities that they are producing plutonium at, they may well decide that their best response is to launch a nuclear missile at Seoul or Tokyo or someplace like that," he said

If Washington were to opt for limited air strikes, it might target the small nuclear reactor at Yongbyon that has been North Korea's sole source of the plutonium so far.

But bombing it would risk creating a large radioactive cloud, Bennett said.

Two larger partially built reactors at Yongbyon and Taechon would be more logical targets if the United States aims to contain North Korea's future plutonium production.

But Michael O'Hanlon, a military expert at the Brookings Institution, said a major military response to the kind of nuclear test that other countries have conducted would not be easy to justify.

"And if we did too much the North Koreans could retaliate and this would lead to an escalation. Once things get going, there's no way of knowing where they'll stop," he said.

earlier related report
US Warns Nkorea Directly Over Nuke Test Plan UN Weighs Response
Washington (AFP) Oct 05 - The United States has warned North Korea directly of the price it will pay if it goes ahead with its threatened nuclear test, as the UN Security Council strove for agreement on a firm response. The US lead negotiator to six-party talks on ending Pyongyang's nuclear weapons program, gave the communist state a stark warning, a day after it announced it would stage a test to deter perceived US aggression.

North Korea has come to "a very important fork in the road -- it can have a future or it can have these (nuclear) weapons but it cannot have them both," said Assistant Secretary of State Christopher Hill.

The North's announcement has sparked alarm worldwide, not least in neighbouring South Korea which said any test would mean an end to economic aid worth tens of millions of dollars.

Hill said Wednesday the North Koreans had been contacted through their United Nations mission in New York.

"Yesterday we sent the message directly to the DPRK (North Korea) through the New York channel on our view of what such a test would mean," Hill said.

It is thought to have been the first direct contact between the two countries since North Korea's dramatic Tuesday announcement, which gave no date for the country's first nuclear test.

Hill left the door open for direct talks. "We have spoken consistently that we will meet them bilaterally, but it has to be in the context of the six-party talks," he said.

US ambassador to Seoul Alexander Vershbow has said a bilateral meeting, which the North seeks, is possible if it commits to returning to the six-party forum.

The North, which last year declared itself a nuclear-armed state, has boycotted the talks since November in protest at US efforts to shut down its overseas banking channels.

Hill said North Korea was told that a nuclear blast "would be a very highly provocative act, and the international community cannot be indifferent to that.

"It would invite the prospect of proliferation ... and we have no choice but to act resolutely to make sure the DPRK and every other country understand" the implications, he added.

Hill said talks are being held within the US government and with allies "and I am not prepared at this point to say what we are going to do but I am prepared to say we are not going to wait for a nuclear North Korea, we are not going to accept it."

The six-party talks hosted by China also involve Japan, the two Koreas, Russia and the United States. There were signs of discord between China and the United States on Wednesday as the Security Council sought agreement on a firm response to the test plan.

After morning closed-door consultations, the council's 15 ambassadors tasked their experts to meet again Thursday to peruse a Japanese draft statement urging North Korea "not to undertake such a test and to refrain from any action that might aggravate tension."

The statement also urges Pyongyang to return immediately and unconditionally to the six-party talks.

Kyodo news agency said Japan and the United States had agreed to seek a council resolution imposing sanctions if North Korea went ahead with the blast.

Earlier in the day, US ambassador to the United Nations John Bolton spoke of "division" within the council, suggesting "North Korea's protectors" -- implying Moscow and Beijing -- opposed his call for a tough line against it.

China's UN envoy Wang Guangya took exception.

"We are all concerned about the North Korean announcement. On this issue, everybody is unanimous. ... No one is going to protect them," he said.

Wang urged "less mistrust" between Washington and Pyongyang.

The United States joined a host of nations in urging the North to drop its plan. But US officials avoided talking about ramping up sanctions and stressed the diplomatic route.

The timing of North Korea's next move, as ever, remained a mystery. But two South Korean ministers said its test announcement, issued by its foreign ministry in measured language, was probably not a bluff.

Defense Minister Yoon Kwang-Ung told parliament in Seoul Wednesday the government assumes the North is likely to make good on its threat.

Unification Minister Lee Jong-Seok said Seoul believes that Pyongyang's statement "is aimed at pressuring the United States to change its stance" towards it.

"However, (the government) believes there is a high possibility of a nuclear test if efforts to resume the six-party talks end in failure."

Lee said the South would have to halt economic aid to its impoverished neighbour if it exploded a nuclear device.

A US intelligence official said unusual movement had been detected at one of several suspected test sites in North Korea.

The official, who spoke to AFP on condition of anonymity, said it was difficult to discern how advanced North Korea's test preparations might be.

"The bottom line is they could conduct it with little or no warning."

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