North Korea may have reactivated a plant for reprocessing plutonium for use in nuclear weapons, the UN atomic watchdog said Monday, citing satellite imagery and echoing recent comments from a US think-tank.

"The indications that we have obtained... (are of) activities related to the five-megawatt reactor, expansion of enrichment facilities and activities related to (plutonium) reprocessing," International Atomic Energy Agency head Yukiya Amano said.

"However, as we do not have inspectors on the ground we are only observing through satellite imagery. We cannot say for sure. But we have indications of certain activities through the satellite imagery," Amano told a regular news conference in Vienna.

He said that the indications spotted at the main Yongbyon complex included the "movement of vehicles, steam, discharge of warm waters or transport of material".

The type of plutonium suitable for a nuclear bomb typically needs to be extracted from spent nuclear reactor fuel.

North Korea mothballed the Yongbyon reactor in 2007 under an aid-for-disarmament accord, but began renovating it after its third nuclear test in 2013. It carried test out a fourth on January 6.

The director of US National Intelligence, James Clapper, warned in February that the North could begin recovering plutonium from the reactor's spent fuel "within a matter of weeks to months".

Amano's comments tally with those of the US-Korea Institute at Johns Hopkins University last week.

The think-tank said that recent satellite pictures showed two rail "flatcars" -- loaded with tanks or casks -- near the Yongbyon complex's radiochemical laboratory.

Similar flatcars were seen during reprocessing campaigns in the early 2000s and the casks could be used to supply chemicals for reprocessing, the institute said.

At the same time, exhaust plumes were seen coming from the lab's thermal plant and coal pens adjoining the plant appeared filled to capacity, it said.

In addition, a lack of activity and steam generation at the main reactor suggest it is either not operating or doing so at an extremely low level, the analysts said.

The reactor needs to be shut down in order to discharge the spent fuel for reprocessing.

IAEA inspectors were kicked out of North Korea in 2009. Amano did not say when the activities spotted by satellite took place.