The South Korean military has deployed two radar-equipped Aegis destroyers to the East and West Sea, to better detect missiles launched by North Korea.
A military source in Seoul says the radar systems have a detection range of one-thousand kilometers, and will be able to track missiles in mid-air.
Just after the North's launch of a long-range rocket from the western Dongchang-ri site last December, the Aegis destroyer detected it within 94 seconds.
The ground-based Green Pine radar and the airborne Peace Eye early warning and control aircraft are also keeping a close watch for developments.
Both Seoul and Washington believe that the Kim Jong-un regime has transported the mid-range Musudan missile to the east coast for a possible test launch.