North Korea has positioned what is believed to be a long-range ballistic missile on a launch pad in what could be a preparation for launch, a U.S. counterproliferation official said on Wednesday.

It's possible that this signals an imminent launch, but the exact time frame remains undetermined, the official told Reuters.

He cautioned that North Korea's intentions in placing the missile at Musudan-ri, a site in the east of the country that has been used for previous missile tests, were hard to gauge. They could also decide to pull the plug, he said.

Another U.S. official, also speaking on condition of anonymity, said North Korea had stacked together two stages of what is expected to be a three-stage rocket.

Pyongyang has said it would launch a satellite between April 4-8. Regional powers see the launch as a disguised test of its longest-range missile and a violation of U.N. sanctions forbidding the reclusive state from firing ballistic missiles.

U.S. Secretary of State Hillary Clinton said during a visit to Mexico that a potential North Korean launch would deal a blow to six-party international talks aimed at reining in Pyongyang's nuclear program.

We have made it very clear that the North Koreans pursue this pathway at a cost and with consequences to the six-party talks which we would like to see revived and moving forward as quickly as possible, she told reporters.

This provocative action ... will not go unnoticed and there will be consequences, she said.

Clinton did not specify the consequences but repeated earlier warnings that a launch could put the issue before the U.N. Security Council for discussions on additional sanctions.

North Korea has given international agencies notice of the rocket's planned trajectory that would take it over Japan, dropping booster stages to its east and west.

Analysts said the notice was given to help the North argue that the rocket launch does not violate U.N. sanctions put in place after it test-launched a series of missiles in 2006.

South Korea, Japan and the United States have all said they want to press for more sanctions against the North for a launch and see no difference between a satellite launch and a missile launch because they use the same rocket -- the Taepodong-2.

The first and only time the North test-launched the Taepodong-2 in 2006, it fizzled shortly into flight and blew apart after about 40 seconds.

Even though the North Koreans have made a public declaration that this is a space launch, it would be in violation of the U.N. Security Council Resolution 1718, said Pentagon press secretary Geoff Morrell. Therefore, we would, of course, oppose it.

He declined to say if the United States would take military action if the missile was launched.

Admiral Timothy Keating, the head of U.S. Pacific Command, has said the United States military could with high probability intercept any North Korean missile heading for U.S. territory, if ordered to do so.

The North's Taepodong-2 missile has a potential range that could take it to Alaska. But analysts do not expect the United States to intercept the rocket, a move Pyongyang has said it would consider an act of war.

Experts have said North Korea would need seven to 10 days preparation once the rocket was on the launch pad before it could be fired. It is visible to spy satellites once it has been removed from an assembly facility and placed on the pad.

Pyongyang said on March 24 that any attempt by the U.N. Security Council to punish it for trying to put a satellite in space would mean the collapse of the international disarmament talks aimed at ending its nuclear program.

(Editing by Chris Wilson)