A Japanese nuclear plant leaked 1.8 tonnes of radioactive water from its cooling system, the government said, heightening safety worries as an atomic crisis continues at another plant.

The leak, discovered Friday, caused no environmental impact as it was contained within an idled reactor at the Genkai nuclear plant in Saga prefecture in the southern Kyushu region, officials said.

Workers are still scrambling to contain a separate ongoing crisis at the Fukushima No. 1 nuclear plant, triggered by the March earthquake and tsunami that hit northeastern Japan.

The operator of the Genkai plant, Kyushu Electric Power, said Friday that one of the water pumps connected to its number three reactor was taken offline after it sounded an alarm for increasing temperature.

But the utility did not announce leaked water at that time. The water kept around the pump and was later collected, said an official with the Nuclear and Industry Safety Agency.

The utility was not legally required to report the water leak, but the mayor of the small Genkai town hosting the plant voiced concerns.

"It should have reported properly" to the Genkai town and Saga prefecture, mayor Hideo Kishimoto told local media, according to Kyodo News.

Officials with the utility could not immediately be reached.

The utility's officials told newspaper the Mainichi Shimbun that water leaks can take place even in a safe operating environment.

Public concerns about nuclear safety came to the fore after the Fukushima crisis forced evacuations of tens of thousands of residents and left large areas of land uninhabitable.

The March disaster knocked out the Fukushima plant's cooling system and sent some of its reactors into meltdown, spewing radiation into the air, sea and food chain in the worst nuclear accident since the 1986 Chernobyl disaster.

That plant's operator said Thursday it was considering dumping more nuclear-contaminated water from the Fukushima power plant into the sea.