The operator of Japan's wrecked Fukushima nuclear plant has taken the first step in the long and hazardous process of decommissioning the facility, extracting four fuel-rods from their container for later removal.

Tokyo Electric Power Co, known as Tepco, said it transferred the uranium and plutonium rods to a steel cask within the same cooling pool in a badly damaged reactor building, beginning on Monday the delicate and unprecedented task of removing 400 tons of highly irradiated spent fuel from that reactor.

"We will continue with the work from tomorrow and proceed, paying close attention to safety," Tepco said in a statement.

The removal of the rods was the most difficult and dangerous task undertaken since runaway reactors were brought under control two years ago.

It follows months of setbacks and glitches that have stoked widespread criticism of the utility's handling of the crisis, the worst nuclear accident in a generation.

The work pales in comparison with the much more complex task that awaits engineers, who will have to remove the misshapen cores of three reactors that went into meltdown.

More than 1,500 rods must be pulled out of the storage pool where they were being kept when a tsunami smashed into Fukushima in March 2011.

Over the course of two days, the company said it expects to remove 22 rods, with the entire operation scheduled to run for more than a year.

A huge crane with a remotely controlled grabber will be lowered into the pool and hook onto the rods, placing them inside a fully immersed cask.

Complicated decommissioning

The 91-ton cask will then be hauled from the pool - to be loaded onto a trailer and taken to a different storage pool about 100 metres away.

Experts have warned that slip-ups could quickly cause the situation to deteriorate. Even minor mishaps will create considerable delays to the already long and complicated decommissioning.

While such operations are routine at other nuclear plants, the disaster has made conditions far more complex, TEPCO has said.

Months of setbacks at the plant have included multiple leaks from tanks storing radioactive water, and a power outage caused when a rat electrocuted itself on a circuit board.

Tepco's management of the problems has been criticised as haphazard and uncoordinated, with one government minister saying it was like watching someone playing "whack-a-mole".

The full decommissioning of Fukushima is likely to take decades and include tasks that have never been attempted anywhere in the world.

Villages and towns nearby remain largely empty. Fear of radiation makes residents unable or unwilling to return to live in the shadow of the leaking plant.