Thirteen years after an earthquake and tsunami hit Fukushima-Daiichi in one of the world's worst nuclear accidents, around 880 tons of extremely hazardous material remain inside.

Late last month, Tokyo Electric Power Company (Tepco) had to suspend a removal procedure after a technical problem involving the installation of the necessary equipment.

"It will take several days for us to prepare for a resumption... and we will be able to resume next week if all goes well as scheduled," a Tepco spokesman told reporters.

Three of Fukushima-Daiichi's six reactors were operating when a tsunami hit on March 11, 2011, knocking down cooling systems and sending them into meltdown in what became the worst nuclear disaster since Chernobyl.

In three units of the Fukushima plant, fuel and other material melted and then solidified into highly radioactive "fuel debris".

The new operation aims to remove a sample of the debris and study it in order to decide on the next steps.

Tepco deployed two mini-drones and a "snake-shaped robot" inside in February, as part of the preparations for removal.

The debris has radiation levels so high that Tepco had to develop specialised robots able to function inside.

Last year, Tepco began releasing into the Pacific Ocean 540 Olympic swimming pools' worth of water used to cool the stricken reactors.

But removing the debris has long been dubbed the most daunting challenge in the decades-long project to decommission the plant.