The operator of the troubled Fukushima Daiichi nuclear power plant says the melted fuel debris likely created holes in the pressure vessel of one of its reactors.

Tokyo Electric Power Company, or TEPCO, on Monday reported their findings to a meeting of the Nuclear Regulation Authority.

The No.1 reactor and two others suffered meltdowns in the 2011 earthquake and tsunami. The containment vessels are filled with water to cool down the fuel.

TEPCO officials said a video taken by an underwater robot shows that a device installed in the reactor pressure vessel's bottom is missing, and there is a dark space where the bottom should be.

The officials also said cooling water is cascading down towards the spot just under the bottom.

They added that the devices that remain inside the reactor appear to be covered with fuel debris.

The officials said these findings led them to conclude that heat from fuel debris created the holes after cooling failed after the tsunami.

TEPCO says it will analyze the footage further.