Foreign Minister Yun Byung-se said his country had asked China and Russia to attempt to persuade the North to soften its hardened stance and abandon any plans for a launch.
Officials in Seoul said surveillance of North Korean activity had been boosted. Missile transporters had been spotted in South Hamgyong province along North Korea's east coast, a rumoured spot for a launch.
"According to intelligence obtained by our side and the US, the possibility of a missile launch by North Korea is very high," Yun told the South Korean parliament.
Through her tears, a North Korean woman told me how she had to escape: risk death while running across the border or die of starvation.
Memories of the Korean war are never far away in South Korea, where residents have grown used to living with the constant threat of war.
North Korea said it will withdraw workers from the Kaesong factory park run jointly with South Korea and suspend all operations there.