North Korea has positioned what is believed to be a Taepodong-2 long-range ballistic missile on its launch pad at a site in the east of the country, Japan's Kyodo news agency reported.
U.S. officials would not confirm the report but did not dispute it. Kyodo, citing "sources close to Japan-U.S. relations," said the missile had been placed at Musudan-ri, a site that has been used for previous missile tests.
"I'm not going to steer you away from what's been reported," a U.S. counter-proliferation official told Reuters.
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.
North Korea 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.
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 the rocket launch does not violate U.N. sanctions put in place after it test-launched a series of missiles in 2006.
"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," Pentagon press secretary Geoff Morrell said.
"Therefore, we would, of course, oppose it."
Morrell 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 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 international disarmament talks aimed at ending its nuclear program.
South Korea, Japan and the United States have all said they want to press 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.