North Korea may fire off a long-range missile in the next two months, military sources in Seoul said Tuesday, amid speculation that the communist nation is in the final stage of preparations for a launch.

Images taken by U.S. satellites showed a marked increase in activity at a North Korean missile launch site in its northwestern tip, suggesting that the country is stepping up preparations for a long-range ballistic missile test.

Last week, Japan's Asahi Shimbun reported the U.S. satellites had picked up images of cargoes carrying what are believed to be missile-related materials toward the Dongchang-ri launch site early this month.

"After the long-range missile parts were transported to the Dongchang-ri missile launch site early this month, apparent signs of preparations for a missile launch were spotted," a senior military official said, asking for anonymity.

"(The South Korean military) is judging that there is high possibility of (the North's) firing off the missile between December and January of next year," the official said.

His remark corresponds with forecasts by the U.S. satellite operator DigitalGlobe Inc., which said Monday that it is possible that Pyongyang could conduct a missile test in the next three weeks.

In recent weeks, North Korea has been stepping up propaganda rhetoric in an apparent attempt to influence South Korea's presidential election, and there have been concerns Pyongyang could stage fresh provocations to affect the polls.

On Tuesday, Defense Minister Kim Kwan-jin held a meeting with 150 commanders and senior military officials to review North Korea's missile preparations and the latest developments of the communist nation.

During the biannual meeting, Kim told the commanders to maintain tight security to deter any threats from the North.

"After the power transition, (North Korean leader) Kim Jong-un will continue to strengthen his grip on power and maintain his hard-line stance on the South," Minister Kim was quoted as saying during the closed-door meeting. "(The regime) will continue to intensify control and monitoring to quell complaints at home."

Cooperation between the two Koreas can come into play only when Seoul's military takes the initiative, the minister said, ordering retaliation against threats from the North. 

Under the new leader, Pyongyang unsuccessfully launched a rocket in April, which was widely seen as a cover for a long-range missile test. (Yonhap News)