The announcement came as Japan opened an emergency center to prepare for the launch. Japan, which along with other regional powers believes the North plans to test a long-range ballistic missile, said last week it would deploy missile interceptors to safeguard against any threat from the launch.
The news agency cited South Korean officials as saying the North had deployed Soviet-made MiG-23 Flogger fighters to guard the rocket, which Pyongyang says will put a communication satellite into orbit.
On Tuesday the North's official news agency warned that if Japan intercepts the rocket launch, expected between April 4 and 8, it will "consider this the start of Japan's war of re-invasion more than six decades after the Second World War, and mercilessly destroy all its interceptor means and citadels with the most powerful military means."
Yonhap agency said the South Korean Air Force is now closely monitoring the movement of jets around the North's Musudan-ri launch site.
Japanese Chief Cabinet Secretary Takeo Kawamura said on Thursday that if North Korea goes ahead with the launch, Japan could halt all exports from the country and tighten restrictions on financial transactions.
However, Russia and China have urged countries to show restraint, and avoid actions that could escalate tensions in the region.
U.S. media quoted senior officials on Thursday as saying the North had begun fuelling the rocket, suggesting that it will be ready for lift off in three or four days' time.
