The announcement was made by U.S. Navy Adm. Samuel J. Locklear III, who repeated earlier U.S. warnings that any satellite-tipped rocket launch the isolated Communist country has said would occur between next Monday and Dec. 22 would be in violation of United Nations Security Council resolutions, the Defense Department said on its web site.
"We encourage the leadership in North Korea to consider what they are doing here and the implications on the overall security environment on the Korean Peninsula, as well as in Asia," Locklear said.
Japan's Kyodo News said the U.S. step involves dispatching two destroyers equipped with the Aegis combat system working with similar Aegis-equipped destroyers of Japan's Maritime Self-Defense Force to track the trajectory of the rocket.
CNN reported the U.S. guided-missile destroyers being moved into place are the USS Benfold and the USS Fitzgerald, but said the Navy declined to give their exact location. Two additional ships may be sent in the next few days, CNN quoted officials as saying.
The Pentagon said North Korea, which claims to have conducted nuclear device tests in the past, has been pursuing nuclear technology and that the launching the rocket could show it has a delivery system for a nuclear weapon.
"And this ... would be very destabilizing ... not only to the region but to the international security environment," Locklear said.
Russia already has urged North Korea not to proceed with the rocket launch. China, North Korea's main ally, also has expressed concern about the launch, saying while the North has a right to peaceful uses of outer space, that right should be exercised within limitation of the U.N. Security Council resolutions.
Earlier, South Korea's Yonhap News Agency, quoting a Seoul government source said, said North Korea has assembled all three stages of its long-range rocket for the launch of its Unha-3 rocket between Dec. 10 and Dec. 22 from the Sohae Space Center in North Phyongan province.
The North has said it would be to place an "Earth observation satellite" into orbit, but critics including the United States suspect it is actually planning a test of its intercontinental ballistic missiles capable of carrying nuclear warheads. A similar attempt in April failed.
The North is prohibited under U.N. Security Council resolution from conducting further nuclear tests or ballistic missile launches.
Yonhap quoted South Korean officials that the next step for the North would be to install support equipment, such as radar and cameras prior to fueling the rocket for launch.
As part of its readiness for the North's launch, Japan also plans to deploy its ground-based Patriot Advanced Capability-3 missile interceptors on Okinawa.
The South Korean military reportedly plans to deploy its new, Israel-made "Green Pine" missile defense radars following their acceptance testing to guard against North Korea.
The North is understood to have notified the International Maritime Organization as well as Japan and other countries within the projected path of its scheduled rocket firing. Yonhap said coordinates provided by the North showed the rocket's first stage would fall into the Yellow Sea between the Korean Peninsula and China, and the second stage drop-off would take place off the Philippines.
"There have been ... a number of signs that might lead you to believe that the new regime leadership is going to take a more ... rational approach to how they deal with their own economy and how they deal with their own people, and how they deal internationally," Locklear said. "There's been a feeling that there might be some hope there."
The North is now led by the young but relatively unknown Kim Jong Un, who took over after the death of his father last December. Since then, the new leader has been consolidating his power.
The Pentagon said Admiral Locklear, whose priority is the defense of the United States, is watching North Korean preparations carefully and talking with friends and allies in the region. The Pacific Command has a homeland defense mission for Guam, the Marianas islands and other states in Oceana, the report said.
"I don't believe that a (North Korean) launch is imminently imminent, but what we are seeing is sort of what they call the beginning of the end of launch preparations," Allison Puccioni, an analyst with Jane's IHS who writes about North Korean imagery for Jane's Defense Weekly, said last week, CNN reported.
"Prior to this, we had not seen much in the way of rocket launch pad activity, but now we are seeing some significant stuff happening," such as additional work on the rocket launch tower, she said.