Cooling system at a reactor of the crippled Fukushima No.1 nuclear power plant stopped from Saturday, the plant's operator said Sunday.

Tokyo Electric Power Co. said the pumps to cool the nuclear reactor and fuel pool have stopped at the No. 5 unit, and the operator is now working to switch to backup pumps to restore the cooling system.

When a worker at the plant became aware of the problem at 9 p.m. Saturday, the temperature of the reactor stood at 68 degrees Celsius and that of the fuel pool at 41 degrees Celsius.

They had risen to 87 degrees Celsius and 44 degrees Celsius, respectively, by the time TEPCO began work to restore operations shortly after 8 a.m. Sunday, it said.