North Korea is far along in its preparations for a test of a long-range ballistic missile and the United States hopes the North Koreans will "give it up and not launch," a senior U.S. official said on Thursday.

White House national security adviser Stephen Hadley told reporters it remained uncertain if North Korea actually planned to conduct a test-firing of the missile, which Washington has warned would be seen as a provocative act.

"I think what we've said publicly is we're watching it very carefully and preparations are very far along. So you could, from a capability standpoint, have a launch. Now what they intend to do ... of course we don't know. What we hope they will do is give it up and not launch," he said.

U.S. defense officials in Washington on Tuesday said the United States has activated its ground-based interceptor missile-defence system amid concerns over the possibility of a North Korean missile launch.

Hadley said the system is basically a research and development project with a "limited operational capability" and that its purpose would be to "defend the territory of the United States from attack," a possible indication it would not be used to try to intercept a North Korean missile fired with an expected landing in the ocean.

Hadley repeated U.S. appeals that North Korea stick with its 1999 moratorium against such launches and return to six-party talks aimed at persuading Pyonyang to give up its nuclear programs.

"We think diplomacy is the right answer and that is what we are pursuing," he said.

He said he could not interpret North Korea's motivations.

"This is a very opaque society and very hard to read. I could speculate on intentions. I don't think it would be useful. What we need to do is look at their capabilities and that's what we're trying to do," he said.