All three stages of the rocket had been assembled on the launch pad in preparation for North Korea's previously announced plans to launch its Unha-3 rocket between Dec. 10 and Dec. 22 from the Sohae Space Center in North Phyongan province to place an "Earth observation satellite" into orbit. A similar launch attempt in April failed.
Critics including the United States suspect the North is actually planning a test of its intercontinental ballistic missiles capable of carrying nuclear warheads.
The United States has said the launch would be a highly provocative act that would violate U.N. Security Council resolutions prohibiting Pyongyang from conducting further nuclear tests or ballistic missile launches.
"North Korea is believed to have completed the installation of a long-range rocket on the launch pad" at the Dongchang-ri base in the country's northwest, a South Korea government official told Yonhap. "Some workers are pulling out of the site."
South Korean officials said the next step for the North would be to install support equipment, such as radar and cameras prior to fueling the rocket for launch.
North Korea is now led by the young and relatively unknown Kim Jong Un, who took over after the December death of his father and is seeking to consolidate his power.
Earlier, Yonhap, quoting a diplomatic source in Seoul, reported that prior to its public announcement last Saturday, North Korea had notified Japan and other countries within the projected path of its scheduled rocket firing.
On Monday, North Korea also reportedly notified the U.N. shipping agency, the International Maritime Organization, about the launch.
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.
Japan already has planned to deploy its ground-based Patriot Advanced Capability-3 missile interceptors ahead of North Korean launch, Kyodo News had reported. The interceptors would be placed at several sites on Okinawa.
Yonhap, meanwhile, quoted a South Korean military source that the country would deploy its new, Israel-made "Green Pine" missile defense radars following their acceptance testing to guard against North Korea.
Russia already has urged North Korea not to proceed with the rocket launch, the Voice of America reported.
China, North Korea's main ally, also expressed concern about the launch. Its Foreign Ministry spokesman Qin Gang said while North Korea has a right to peaceful uses of outer space, that right should be exercised within limitation of the U.N. Security Council resolutions.
In the original announcement, carried by the official Korean Central News Agency, a spokesman for the North Korean space technology said: "Scientists and technicians of [North Korea] analyzed the mistakes that were made during the previous April launch and deepened the work of improving the reliability and precision of the satellite and carrier rocket. The long-range rocket in April crashed into the sea after traveling a short distance, the report said.
The U.N. Security Council had condemned the April launch, demanding North Korea fully comply with its resolutions and suspend all activities related to its ballistic missile program.