South Korea's spy service believes North Korea is preparing for a fourth nuclear test but not in the immediate future.

The office of lawmaker Shin Kyung-min says the National Intelligence Service made the assessment in a closed-door meeting with lawmakers Tuesday.

A memo provided by Shin's office cited the agency as saying it has been monitoring activities at the North's main Nyongbyon nuclear complex using human and technical intelligence assets.

Last month North Korea said it had upgraded and restarted all of its atomic fuel plants, sparking speculation that it might conduct a fourth nuclear test explosion.

The speculation subsided after North Korea didn't go ahead with a threat to implement a banned long-range rocket test. All of the North's previous three nuclear tests came after it launched long-range rockets.