Pacific forces commander Adm. Samuel Locklear said it was unclear whether North Korea has corrected the problems of a failed launch of a similar long-range rocket in April that drew U.N. condemnation. He said such a missile capability would be destabilizing to international security.
North Korea says it has only peaceful intentions. It says the launch will take place between Dec. 10 and Dec. 22.
In Seoul, a South Korean military intelligence official said Wednesday that North Korea has mounted the first and second stages of a three-stage rocket on the Sohae launch pad on its northwest coast. South Korean officials couldn't confirm media reports that all three stages of the rocket are in place.
North Korea has a long history of developing ballistic missiles, but in four attempts since 1998 has not successfully completed the launch of a three-stage rocket. It has also conducted two nuclear tests, intensifying concern over how its rocket technology could be used in the future, particularly if it masters how to attach a nuclear warhead to a missile.
Locklear told a Pentagon news conference that North Korea wants to "demonstrate to the world that they have the capacity to be able to build missiles and have the missile technology to be able to use it in ways of their choosing down the road."
"This would be very destabilizing not only to the region, but to the international security environment," he said.
Locklear said the U.S. is moving ships to the region to have the best "situational awareness"—and to reassure allies.
Two U.S. officials said Wednesday that no more than three or four ships, with ballistic missile defense capabilities, are being repositioned to the Western Pacific. The officials spoke on condition of anonymity because they were not authorized to speak publicly about ship movements.
In Tokyo, Lt. Gen. Salvatore Angelella, the commander of American troops in Japan said Thursday that the situation ahead of the planned launch is "very dangerous." He said American troops are working closely with the Japanese to protect the country's citizens and territory, but declined to give details.
The U.S., Japan and South Korea say they'll seek U.N. Security Council action if the launch goes ahead in defiance of existing resolutions. Key to the world body's endorsing any further punishments will be winning the support of China, which is North Korea's main ally and economic partner, and Russia.
The council condemned April's launch and ordered seizure of assets of three North Korean state companies linked to financing, exporting and procuring weapons and missile technology.
Associated Press writers Pauline Jelinek and Lolita C. Baldor in Washington and Hyung-jin Kim in Seoul, South Korea, contributed to this report.