The operator of the damaged Fukushima Daiichi nuclear power plant has released images taken by a robot probe underneath the facility's No.1 reactor.

The pictures captured by Tokyo Electric Power Company, or TEPCO, show what is thought to be nuclear-fuel debris. It's the first robot survey of the section since the 2011 accident.

Accumulated matter can be seen inside a structure that supports the reactor. It appears to be nuclear-fuel debris -- a mixture of molten nuclear fuel and other materials.

The accident caused meltdowns in three of the Daiichi plant's reactors. An estimated 880 tons of fuel debris remains inside. Removing it is seen as the biggest challenge in decommissioning the plant. But it's still unclear how workers will do it.

Other images show that reinforcing steel bars in the reactor's supporting structure are exposed.

TEPCO officials say at least half of the structure was severely damaged. They say they will also study the level of earthquake resistance.