The operator of Japan's crippled Fukushima nuclear plant said Tuesday it had shut down a key decontamination system used to clean radiation-tainted water, just hours after it came back online.

Tokyo Electric Power (TEPCO) switched off its Advanced Liquid Processing System (ALPS) after workers discovered leaks "seeping" from a tank late Monday.

About eight litres (2.1 gallons) of tainted water is believed to have leaked out, a company spokesman said. He added there was no immediate safety risk as the water had been recovered.

The suspension came just six hours after the embattled operator switched back on two of three lines in the system, which cleans radiation-tainted water used to cool the reactors damaged by Japan's devastating 2011 quake-tsunami disaster.

The whole system had been shut down last Wednesday after TEPCO discovered a defect. The firm has repeatedly switched the system off over a series of glitches since trial operations began a year ago.

TEPCO is struggling to handle a huge -- and growing -- volume of contaminated water at Fukushima following the quake-tsunami, which sparked the worst atomic crisis in a generation as well as sweeping away some 18,000 victims.

Giant waves crashed into the plant, sparking reactor meltdowns and explosions. Cleaning up the shattered site is expected to take decades.

There are about 436,000 cubic metres of contaminated water stored at the site in about 1,200 purpose-built tanks.

Many experts say that at some point the water will have to be released into the sea after being scrubbed of the most harmful contaminants.

They say it will pose a negligible risk to marine life or people, but local fishermen and neighbouring countries are fiercely opposed.