The operator of the Fukushima Daiichi nuclear plant says a crane malfunction forced it to suspend the removal of fuel assemblies from the spent fuel pool of reactor 4.

The high-risk operation is a crucial part of the process to decommission the facility. It involves transferring fuel assemblies into a special cask and transporting them to a more secure location.

Tokyo Electric Power Company (TEPCO) officials say the crane used to handle the cask ceased operating at around 9:30AM on Wednesday.

They say the fuel assemblies inside the pool do not show any damage. Officials are investigating the exact cause of the malfunction.

The presence of 1,533 fuel assemblies in the pool triggered international concerns after the triple meltdown of March 2011. Some experts called for their immediate removal, saying they posed a significant risk in the event of another earthquake.

The operation began in late November 2013 and is scheduled to be completed by the end of this year. 528 assemblies have been removed so far.