Activity has been spotted at a launch station on the west coast of the isolated nation. Earlier this week Pyongyang announced it was planning to launch a satellite at some point in February.
The announcement was internationally condemned - critics say it is a cover to test banned missile technology.
North Korea also conducted its fourth nuclear bomb test on 6 January. UN sanctions against North Korea prohibit it from carrying out any nuclear or ballistic missile tests.
South Korean state news agency Yonhap reported defence ministry officials on Thursday as saying activity had been spotted at a site in Dongchang-ri, where the Sohae launching station is located.
