North Korea may be attempting to restart its main nuclear bomb fuel reactor after a five-month shutdown, experts believe.

Newly released satellite images suggest the country could be in the early stages of bringing its Yongbyon reactor complex back into use.

When fully operational, the five-megawatt facility is capable of producing about six kilos of plutonium a year - enough for a nuclear bomb.

Scroll down for video

Satellite images (above) taken on January 2 suggest North Korea could be in the early stages of bringing its Yongbyon reactor complex back into use

North Korea has conducted three nuclear tests, and recently threatened a fourth, amid tensions over fresh US sanctions and UN moves to censure Pyongyang for its human rights record.

The US-Korea Institute at Johns Hopkins University said the latest satellite images showed fresh activity at Yongbyon prompting suggestions of 'the early stages of an effort to restart the reactor'.

Signs included steam coming from a probable pressure relief valve on a steam pipe just before it enters the turbine building, and meltwater running off the centre of the turbine-building roof, the institute said in a post on its 38 North website.

The analysis stressed that the activity was too 'limited' to enable an accurate assessment of when the reactor might be tested and become operational.

The think-tank said there were clear differences between the latest 2014-1015 imagery and that from more than a year earlier when the reactor was known to have been operating.

North Korea shut down the Yongbyon reactor in 2007 under an aid-for-disarmament accord, but began renovating it after its last nuclear test in 2013. The country's leader Kim Jong Un is pictured earlier this month

When fully operational, the five-megawatt facility is capable of producing about six kilos (13 pounds) of plutonium a year - enough for one nuclear bomb

Imagery from December 2013 showed snow had melted off the roofs of all the buildings related to the reactor and foam could be seen at the end of the turbine building's steam and wastewater drainpipe.

It said the absence of the foam in recent images could be related to the installation of new piping.

North Korea announced in April 2013 that it would revive the aged five-megawatt research reactor at the Yongbyon nuclear complex, saying it was seeking a deterrent capacity, a move condemned by members states of the U.N. nuclear watchdog, the International Atomic Energy Agency.

North Korea shut down the Yongbyon reactor in 2007 under an aid-for-disarmament accord, but began renovating it after its last nuclear test in 2013.

The UN's nuclear watchdog, the International Atomic Energy Agency, said in September last year that steam discharges suggested the reactor might already be operational again.

But analysis of later satellite imagery led other experts to conclude it had either failed to restart properly or had been shut down again - possibly for refuelling or renovation.

Unused fuel rods are pictured in 2009 stacked in a warehouse at North Korea's nuclear complex in Yongbyon

Pictures have also detected activity at a separate Yongbyon facility, thought to be used for enriching uranium.

The North says the purpose is to produce low-enriched uranium for a new reactor it is constructing, but experts suspect that the real goal is weapons-grade uranium, an alternative to plutonium for a nuclear bomb.

Earlier this month, North Korea offered to suspend nuclear tests in exchange for a temporary freeze on US-South Korea joint military exercises.

Washington rejected the offer, which it described as an 'implicit threat' to detonate a fourth nuclear device.