South Korea's spy agency confirmed Tuesday that the North has restarted an aging plutonium reactor that could help boost its nuclear weapons programme.

The National Intelligence Service (NIS) said in a report to parliament that the five megawatt reactor at the North's Yongbyon nuclear complex had resumed operations, according to a joint briefing by ruling and opposition party lawmakers.

The report was presented at a closed intelligence committee session, lawmakers told media.

The spy agency declined to comment on the report.

It followed speculation by the US-Korea Institute at Johns Hopkins University in Baltimore that North Korea had restarted the reactor.

A commercial satellite image taken on September 19 showed the plutonium reactor releasing hot waste water into a river through a new drainpipe, the think tank said last week.

An image from late July had not shown any sign of hot water discharge, however, indicating a recent relaunch.

In reports released last month, the institute and another Washington think tank, the Institute for Science and International Security, observed steam coming from the reactor.

The drainpipe is critical to maintaining a safe temperature at the reactor. North Korea knocked down a cooling tower in 2008 to show its commitment to a US-backed aid-for-disarmament deal.

Pyongyang carried out its third nuclear test in February, sparking international condemnation and raising tensions on the Korean peninsula for months.

Two months later, it boasted that it would restart all facilities at Yongbyon to bolster its atomic arsenal.

Russia has warned that the resumption of Yongbyon could lead to catastrophe. The reactor, a source of great national pride and international anxiety over its role at the heart of North Korea's nuclear ambitions, was built in 1986 and is outdated.

"Our main concern is linked to a very likely man-made disaster as a consequence. The reactor is in a nightmarish state, it is a design dating back to the 1950s," a Russian diplomatic source told Interfax news agency last month.

"For the Korean peninsula this could entail terrible consequences, if not a man-made catastrophe."

The United States and South Korea have also voiced concern, with evidence of the reactor's restart reinforcing scepticism over North Korea's statements that the communist state is ready to return to negotiations.

The NIS report also showed that the North had tested a long-range rocket engine, the lawmakers said.

The US-Korea Institute, citing satellite images, said separately in September that North Korea was believed to have tested a long-range rocket engine at its Sohae satellite launching station.

While the exact engine type could not be identified, possibilities included the second stage of the Unha-3 Space Launch Vehicle, or the second or third stage engine of a much larger rocket under development, it said.

Analysis of before and after satellite photos indicated the test had taken place sometime between August 25 and 30, it said.

Sohae was the base for the successful launch of the Unha-3 rocket in December -- an event condemned by the West as a disguised long-range ballistic missile test that violated UN resolutions.

The UN Security Council tightened sanctions against North Korea after the launch -- and then again after the North's missile test in February.