Image copyright AFP Image caption Lava-like rocks were spotted underneath one of the damaged reactors

An underwater robot has captured what is believed to be the first images of melted nuclear fuel deposits inside Japan's stricken Fukushima nuclear plant, its operator Tepco says.

Large amounts of solidified lava-like rocks and lumps in layers were seen underneath its unit three reactor.

If confirmed, it would be a major milestone in the clear-up operation.

The power plant was hit by a tsunami in 2011, causing the most serious nuclear accident since Chernobyl.

More than 200,000 people were forced to leave their homes because of nuclear contamination concerns when three reactors failed at the plant after it was flooded.

Parts of the damaged reactors are still highly contaminated, with radiation and robotics playing a crucial part in the clean-up.

Locating the fuel debris in each of the damaged reactors is an important part of the decommissioning process, which is expected to take decades.

Tokyo Electric Power Company (Tepco) said that the images - revealed on Friday - were the first "high likely" sighting of melted fuel since the 2011 disaster.

Image copyright AFP Image caption The lower part of a control rod mechanism inside reactor unit three

"There is a high possibility that the solidified objects are mixtures of melted metal and fuel that fell from the vessel," a spokesman said.

Some of the objects appeared like icicles hanging around a control rod mechanism, which is attached to the bottom of the reactor's pressure vessel holding the core, the company said.

According to the Japan Times, fuel rod assemblies contained in the pressure vessel melted into a puddle and burned through the bottom after the tsunami six years ago.

Lumps of material that appeared to have melted and re-solidified near the wall of the pedestal, a concrete structure supporting the pressure vessel, were also captured on camera.

Image copyright AFP Image caption Part of the pedestal wall inside the same reactor

The company said more time was needed to analyse the debris further.

The deposits were found underneath the core inside the primary containment vessel of Fukushima's unit 3 reactor.

Image copyright (C) British Broadcasting Corporation Image caption How Japan managed to get Fukushima's reactors stable during the disaster

The recent findings followed a three-day investigation using a small, remote-controlled underwater robot nicknamed "Little Sunfish".

More than 18,500 people were killed or went missing when an earthquake triggered the massive tsunami that led to the Fukushima meltdown in 2011.

No-one died directly in the Fukushima disaster. However, three former Tepco executives are currently on trial on charges of negligence resulting in death and injury of more than 40 patients evacuated from a hospital near the plant.