Shin Won-sik told local media Thursday that the new reactor at North Korea's Yongbyon nuclear complex appears to be in a "trial operation" stage and that it is expected to be fully operational next summer.

The defence ministry confirmed Shin's comments Friday.

Yongbyon is home to North Korea's only other nuclear reactor, with a five-megawatt capacity, which has been its sole known source of plutonium -- a primary material for nuclear weapons.

Rafael Grossi, head of the UN atomic agency, said in a statement last week that the new light-water reactor appeared to be operational and capable of producing plutonium.

"This is a cause for concern," he said.

Shin noted that while no country has used light-water reactors to produce weapons-grade plutonium, Pyongyang could potentially use the new reactor to produce tritium for hydrogen bombs or to conduct tests for developing small nuclear reactors used in nuclear-powered submarines.

The defence minister said Seoul has detected signs of cooling water discharged from the new reactor since this summer.

Grossi said last week that the new reactor has "reached criticality", and that it was discharging warm water.

"Reaching criticality means that the nuclear reactor operates under normal condition by realizing sustainable nuclear chain reaction," Lee You-ho, a nuclear engineering professor at Seoul National University, told AFP.

"When a nuclear reactor operates under normal condition, it produces heat which accompanies the discharge of warm water."

Shin's comments came the same day that Seoul's spy agency issued a statement forecasting that Pyongyang would carry out military and cyber provocations in the coming year, targeting election campaigns in the United States and South Korea.

North Korean leader Kim Jong Un recently instructed his aides to "come up with measures to cause a big stir in South Korea early next year", according to the statement by Seoul's spy agency.

Kim this week called for the acceleration of war preparations, including in the country's nuclear programme and munitions industry, to counter what he called "unprecedented" anti-North confrontational moves by Washington.