Satellite images released since Tuesday showed the North moving two missile transporter-erector-launchers to the eastern South Hamgyeong province, South Korea's Yonhap News Agency reported.
The move came after Pyongyang previously shifted two mid-range Musudan missiles to the city of Wonsan and put seven mobile missile TELs in Wonsan and South Hamgyeong province.
A South Korean military official, speaking on the condition of anonymity, said Seoul has been monitoring Pyongyang's moves.
"The military is closely watching the North's latest preparations for a missile launch," the officials said.
North Korea recently celebrated the birthday of the current ruler Kim Jong-un's grandfather and founder of the Korean People's Army, Kim Il-sung, and will mark the anniversary of the founding of the party on Thursday. Military officials said the North may try to stage a show of force, such as a missile launch or a military parade, during the anniversary
However, Kim Jang-soo, the South Korean chief of National Security, said last Thursday the North has shown no signs of preparing for a missile launch, adding he does not believe the North is capable of conducting a full-scale war on the Korean Peninsula.