The operator of the crippled Fukushima Daiichi nuclear complex said Friday it found material likely to be nuclear fuel debris in its latest probe at one of the plant's three damaged reactors.

(International Research Institute for Nuclear Decommissioning)

It is the first time material "highly likely" to be melted fuel has been spotted inside a reactor at the complex since the 2011 massive earthquake and tsunami crippled the Fukushima nuclear power plant, the operator Tokyo Electric Power Company Holdings Inc. said.

Material found near the bottom of the damaged pressure vessel of the No. 3 reactor is likely to have been formed by fuel that melted, Takahiro Kimoto, a Tepco spokesman, said at a press conference.

"We think that the fuel inside the vessel melted and caused structures to fall from above," Kimoto said.

Tepco is trying to confirm the condition of fuel debris inside the No. 3 reactor with a robot to start work on removing the debris in 2021, one of the most difficult stages of the decommissioning project that is expected to take at least 30 to 40 years to complete.

Decommissioning work has progressed slowly as radiation levels inside the reactors remain extremely high.

Water around 6.4 meters deep, which was injected into the reactor to cool fuel debris inside, has accumulated at the bottom of the containment vessel.

Tepco used a cylinder-shaped underwater robot with a diameter of 13 centimeters, dubbed "little sunfish."

In Friday's probe, following one on Wednesday, the robot looked inside the reactor's containment vessel housing the pressure vessel, which is partially filled with contaminated water.

Another round of probe is scheduled on Saturday, where the remote-controlled robot is expected to swim deeper to the bottom of the containment vessel to locate a chunk of melted fuel that is believed to lie.

On March 11, 2011, a huge tsunami hit the six-reactor plant, located on ground 10 meters above sea level, and flooded power supply facilities.

Reactor cooling systems were crippled and the Nos. 1-3 units suffered fuel meltdowns in the world's worst nuclear disaster since the 1986 Chernobyl crisis.

From January to March, Tepco conducted robot surveys including sending a self-propelled robot into the Nos. 1 and 2 reactors, where water levels are lower than the No. 3 reactor, but they failed to ascertain the condition of fuel debris.