A South Korean military official says they have detected suspected signs that North Korea is preparing to launch a military spy satellite.
South Korea's Yonhap News Agency says the North may be ready to proceed with the launch as early as next week.
After announcing it successfully launched its first military satellite last November, North Korea revealed plans to launch three more such satellites this year.
In March, South Korean media quoted a military source as saying that North Korea had set up screens at the Tongchang-ri launching station in the country's northwest, likely to hinder monitoring by US reconnaissance satellites.
Early this year, North Korea began describing the South as an "invariable principal enemy," defining it as a hostile nation rather than a potential counterpart for unification.
The suspected preparations for launching the spy satellite come ahead of a three-way summit next week by Japan, China and South Korea.
The South Korean government will likely bring up the subject of North Korea at the summit, as well as in separate bilateral talks with the two countries.
South Korea's military plans to keep working with the United States and other parties to step up vigilance and surveillance as such discussions may provoke a backlash from the North.
Japan's Self-Defense Forces are also on the alert over North Korea's possible launch of a military satellite.
They have deployed Aegis destroyers equipped with high-performance radar and SM-3 interceptor missiles in the East China Sea, and land-based PAC-3 interceptor missile units in Okinawa Prefecture.
Defense ministry sources told NHK that the ministry is aware of North Korea's moves, and is analyzing the details.