South Korea's defense chief says North Korea's experimental light water nuclear reactor may become fully operational next summer.

Defense Minister Shin Won-sik spoke to South Korean media on Thursday about the reactor under construction at the North's Nyongbyon complex. The site also has an experimental graphite-moderated nuclear reactor that can extract plutonium, a key fuel for nuclear weapons.

Shin said Seoul had detected signs of cooling water being discharged from the light water reactor since last summer, and estimated that it would likely be ready for full operation next summer.

Pyongyang says the reactor is for power generation. But Shin points out that if it is for military purposes, it could be used for tests to develop a small reactor for a nuclear-powered submarine. The defense chief stressed that Seoul will closely monitor the North's moves.

North Korea has listed possessing its first nuclear-powered submarine in its five-year defense program.

Last week, International Atomic Energy Agency head Rafael Grossi expressed concern that warm water discharges around the light water reactor indicate that the facility has reached criticality.