South Korea's spy agency has said it is possible North Korea can develop an intercontinental ballistic missile capable of reaching the US mainland this year and that it is monitoring developments closely.

No sign of an imminent nuclear test had been detected, though the North's Punggye-ri complex appears ready for another detonation “at any time”, the agency told lawmakers.

North Korea, pursuing nuclear and missile programmes in defiance of world condemnation, is also enforcing stronger controls on outside information in the face of international sanctions, the lawmakers said after a closed-door briefing.