TOKYO (AFP) - A power failure at Japan's tsunami-crippled Fukushima nuclear plant forced operator Tokyo Electric Power Co (Tepco) to suspend systems to cool spent fuel pools at three reactor units, a report said on Tuesday.

The incident has not so far affected water injection to the number 1 and number 3 reactors, which suffered core meltdowns soon after the start of the March 2011 nuclear crisis, according to the Nuclear Regulation Authority (NRA), Kyodo news agency said.

No major changes have been observed in radioactivity levels detected by nearby monitoring posts after the incident, which happened just before 7.00pm (6pm Singapore time) on Monday.

As of 1.45am Tuesday, Tepco has not been able to bring the system back online, the Kyodo report said.

The problems with electrical power have also led to the suspension of operation of equipment to treat contaminated discharge including radioactive materials.

According to the NRA, Tepco reported to regulators that electricity went out at the plant's accident response centre at about 6.57pm on Monday.

If no cooling steps are taken, the temperature of the water inside the spent fuel tank at the number 4 unit, which has posted the highest temperature, would reach 65 deg C - the upper safety margin limit Tepco has set - within four to five days, the power company said.

The meltdown at the core of three of Fukushima's six reactors occurred after the March 11, 2011, earthquake and ensuing huge tsunami shut off the power supply and cooling system.