"We have a missile defense system . . . that is basically a research, development, training, test kind of system" but it has "some limited operational capability," Hadley told reporters in Budapest, where he was travelling with US President George W. Bush before returning to Washington.

"And the purpose, of course, of a missile defense system is to defend the territory of the United States from attack," Hadley said.

Asked if the United States would shoot down a test missile if North Korea launched one, but not necessarily aimed at US territory, Hadley would only say that he hoped North Korea, which claims to have nuclear weapons, would come back to the negotiating table and heed international warnings.

According to the warning "a North Korean missile test right now would be, again, breach of North Korea's unilateral undertakings."

In Washington, a senior official said North Korea would pay "some cost" if it launched a long-range missile.

"If such a launch takes place, we would seek to impose some cost on North Korea," Peter Rodman, assistant secretary of defense for international security affairs, said during testimony before the House Armed Services Committee.

Preparations for the launch of a multi-stage Taepodong-2 with a range of up to 6,700 kilometers (4,200 miles) have been underway for several weeks at Musudanri on the remote northeast coast of North Korea.

But South Korean Defense Minister Yoon Kwang-ung said Thursday that a launch was not imminent, while another senior Seoul official said the communist North has made no substantial moves for several days towards a firing.

A senior US defense official, meanwhile, said the US military would use any capability it has to protect the American people if North Korea launched a missile at the United States.

The official, however, said the US missile defense system would not necessarily be used if North Korea launched a missile that was headed into open ocean.

His and Hadley's comments were the clearest official indication yet that the United States has activated its missile defense system in the face of North Korean preparations for a launch of a long-range Taepodong missile.

Former US defense secretary William Perry went even further, calling Thursday for an ultimatum for North Korea to defuel and put away the missile or face a US missile strike to destroy it before it can be launched.

North Korean preparations to test launch a missile "are very far along," Hadley said.

But he said it was hard to guess what the North Koreans will actually do.

"Preparations are very far along, so you could, from a capability standpoint, have a launch," Hadley said.

"Now what they intend to do, which is what a lot of people are trying to read, of course, we don't know. What we hope they will do is give it up and not launch," Hadley said.

He said that North Korea "is a very opaque society and very hard to read" and that it was not useful to speculate on Pyongyang's intentions.

"What we need to do is look at their capabilities, and that's what we're trying to do," Hadley said.

He said North Korea had "voluntarily" adopted a moratorium in 1999 on missile testing "and reaffirmed it several years later."

"And our position is that the North Koreans ... should not test, should not test. They should respect their own moratorium," Hadley said.

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