Japan has unveiled an underwater robot to inspect the damage at the Fukushima Daiichi nuclear plant after it was hit by a tsunami and earthquake in March 2011.

The developers plan to deploy the robot into the primary containment vessel of Unit 3 at Fukushima in July to study the damage and locate parts of melted fuel believed to have fallen to the bottom of the chamber.

Tens of thousands of residents evacuated their homes after the 2011 meltdown. Due to high radiation levels residents are still unable to return.

Dubbed mini manbo or little sunfish, the robot weighs around 2kg and has a diameter of 13cm and length of 30cm. It is equipped with front and rear cameras and collects data using its two cameras and a dosimeter.

The swimming robot has been co-developed by Toshiba and the International Research Institute for Nuclear Decommissioning (IRID). During a demonstration at a test facility near Tokyo on Thursday, the robot was remotely operated by a team with one guiding the robot and another adjusting a cable that transmits data, the Associated Press reported.

Officials believe the robot can swim deep to illuminate the area underneath the reactor's core.

"The fuel debris will be a challenge," Dale Klein, a former US Nuclear Regulatory Commission chief told the Associated Press, adding it would take at least six months to a year to collect data and decide on how to remove the fuel.

"They will have to identify where it is, then they will have to develop capability to remove it. No one in the world has ever had to remove material like this before. So this is something new and it would have to be done carefully and accurately," he said.

Japanese officials plan to determine removal methods this summer before they start work in 2021.

"We have already developed remotely operated robots for inspections at Fukushima," Goro Yanase, general manager of Toshiba's Nuclear Energy Systems & Services Div said in a statement issued on Thursday.

"In this case, we had to meet the specific challenges of limited access and flooding, in a highly radioactive environment. Working with IRID, we succeeded in developing a small robot with high level radiation resistance, and through its deployment we expect to get information that will support the advance of decommissioning," Yanase said.