North Korea last weekend announced plans to use a long-range rocket to try to send a satellite into space this month. Similar launch efforts in 2006 and 2009 coincided with nuclear tests and this month's launch could occur before Dec. 19 presidential elections in South Korea.
A government official from South Korea told the Yonhap news agency on condition of anonymity that workers at a launch facility in North Korea completed assembly of a three-stage rocket.
A South Korea missile expert said he believed the rocket was an Unha-3 model that has a range that could reach Los Angeles.
An Unha-2 model flew about 2,300 miles, well short of the U.S. mainland, in 2009.
Yonhap said it expects the North Koreans to launch the rocket as early as Monday. This week, Pyongyang notified the International Maritime Organization of the launch plans.
Early this year, North Korea vowed to halt military and nuclear testing in exchange for food assistance from the United States. It reneged on that pledge, however, with a failed launch attempt in April.