Pyongyang said a day earlier it plans to launch a rocket carrying a "working satellite" between Dec. 10 and 22, with much of the world suspecting it is in reality testing ballistic missiles capable of carrying nuclear warheads.
The planned launch will be the North's second launch attempt under current leader Kim Jong-un, following a failed launch in April. The young leader took power following his father's death last December.
In an attempt to correct errors that caused the failure in April, when the rocket broke apart shortly after liftoff, the communist country "has secretly pushed to invite foreign experts there, and an unidentified expert recently visited Pyongyang," a senior military official in Seoul said, requesting anonymity.
In July 2011, two North Koreans were arrested in Ukraine on charges of trying to steal secret technical information about missile engines.
"North Korea has stepped up activities for the planned launch. We've learned it is working on a booster and communication check at its Dongchang-ri launch site," a Seoul government source said, adding Pyongyang has "yet to erect the booster for the missile."
The planned rocket launch is "a kind of celebratory firework," to mark the first anniversary of its new leader, another senior government official here said.
"The hurried attempt to launch the missile in the midst of winter seems to be caused by internal factors, rather than outside ones," he said. "By firing the rocket, the North aims to parade its leader's achievements and strengthen the solidarity of its people," he said.
Stressing the firm stance of the international community against the North's provocative moves, another high-ranking official from Seoul's presidential office said Pyongyang will "face sanctions not only from the United Nations' Security Council but from individual countries."
"Many nations are considering sanctions different from the past in terms of their scope and content," the official said on condition of anonymity. "We have also discussed with the U.S. about possible countermeasures against the North's rocket launch since last week."
China also has been trying to persuade Pyongyang against its plan, he said, adding Beijing "would not take the North's side as the rocket launch is also against its own security."
In a meeting with a group of South Korean reporters last week, Chinese Foreign Ministry spokesman Hong Lei expressed concern about "any provocations hurting peace and security on the Korean Peninsula."
South Korean and Chinese top nuclear envoys also agreed to strengthen efforts to stabilize the situation and keep close watch on the North during their meeting in Beijing last week.