US national security advisor Stephen Hadley said it was "hard to tell" whether the North had finished fuelling up its Taepodong-2 for a launch that would end a moratorium on its missile tests in place since 1999.

"They seem to be moving forward towards a launch but the intelligence is not conclusive at this point," Hadley told reporters on Air Force One as it took President George W. Bush to a trip to Europe.

Hadley said North Korea appeared to be deliberately seeking to raise tensions.

"There tends to be a desire to create a sense of crisis; they seem to think that's something that works for them. And they've done these kinds of things to get attention before. They did a missile launch in 1998," he said.

White House officials would not comment on what action the United States was considering if North Korea does launch a missile. No country has yet announced what could be done.

"There are a variety of options, we are simply not going to tip our hands," White House spokesman Tony Snow told the briefing.

"Doesn't mean they don't hear it, just means you don't," Hadley said of the options being considered by the United States and other concerned countries.

The Washington Times, citing US officials, reported that a ground-based missile defence system had been switched from test to operational mode in the past two weeks.

The US Department of Defense refused to confirm or deny the report, however.

The Taepodong-2 is believed to have a range of up to 6,700 kilometers (4,200 miles), far enough to hit targets in Alaska and possibly Hawaii.

Japan and the United States have warned of a tough response to any launch, but South Korea has cautioned there is no certainty a launch will go ahead.

North Korea caused alarm in August 1998 by firing a Taepodong-1 missile with a range of up to 2,000 kilometers (1,250 miles) over Japan into the Pacific Ocean. It claimed this was a satellite launch attempt.

The North declared a moratorium on tests of long-range missiles in 1999, but said in 2005 that it would no longer observe it.

South Korea's Foreign Minister Ban Ki-Moon said the only certainty about the latest alert was that there is a missile ready on a launch pad at Musudanri on a remote stretch of coast in the northeast of the Stalinist state.

"The reports say it may be imminent that North Koreans may test-fire a long-range missile but it is not quite sure they have put fuel in the rocket," Ban said on the sidelines of a UN meeting in Geneva.

"What seems sure is that they have assembled this missile on the launching pad," he added.

Ban discussed the latest North Korea alert on Tuesday with US Secretary of State Condoleezza Rice and Japan's Foreign Minister Taro Aso, officials in the three countries said.

The Japanese foreign ministry said that Aso and Ban had agreed the two countries should cooperate to prevent any missile launch, which comes as the international community tries to pressure the North to return to talks over its nuclear weapons programme.

China, North Korea's last ally of consequence, has been the only country not to publicly speak out over the missile reports.

But Washington is satisfied that China is taking action, a US official told reporters on condition of anonymity.

"We are confident and we know that they are engaging with North Koreans in a way that they think is effective to bring about the result that we all want," said the official.

"And if it is doing so in a public statement, that's fine. If they don't want to do it in a public statement, that's fine too. That is not a reflection of the effectiveness of their action," said the US official.

US Warns Of UN, Bilateral Retaliation If N Korean Missile Is Launched

A senior US State Department official said Tuesday that retaliation to a North Korean ballistic missile test could include United Nations as well as bilateral punishment.

The official, speaking to journalists during a trip to Paris on condition he not be named, said that, if North Korea goes ahead with a test of its Taepodong-2 missile, the matter would be referred to the UN Security Council in New York.

"Clearly, there would be action taken in New York, there would be also a series of steps that the United States and other countries could take separately or collectively," he said.

The United States, Japan, Australia, France and other countries have expressed alarm at North Korea's preparations to test-fire the missile, which is believed to be able to carry a one-tonne warhead up to 6,700 kilometres (4,200 miles).

"We find that there is this shared sense that if North Korea takes this action, there should be a strong response," the official said.

He said that, in that case, Washington would look at "using all the tools we have, including the financial tool available".

A top South Korean intelligence official said Tuesday that North Korea appeared to be still fuelling the missile at Musudanri in its remote northeastern province of North Hamgyong.

A report by the New York Times on Monday had quoted a US official as saying that satellite images suggested fuelling was complete and booster rockets had already been fitted to the missile.

The State Department official speaking in Paris said "we don't know when North Korea will actually launch this missile," but warned "if they do launch this will be considered a provocative act not just by the United States but by the broader international community."

He raised the prospect of a regional arms race being sparked, saying that a launch "will have a significant effect on Japan and its security calculations".

Japan, which has long been uneasy about its communist neighbours' intentions and military capability, has been under a US security umbrella since losing World War II.

Asked about reports that the missile had a range capable of striking the continental United States, the official said "we don't really know the outer limits" of the missile.

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