A nuclear power reactor was shut down automatically in western Japan on Tuesday, but the cause of the suspension was not immediately known, its operator said.

Operations at the number four reactor of Genkai Nuclear Power Plant in Saga were automatically suspended at around 1:40 pm (0440 GMT), said Kyushu Electric Power, which runs the plant.

"We have not monitored any abnormality such as any change in radiation gauges," Yuki Hirano, a company spokesman, said, adding that there were no reports of fire or smoke.

The operator said that an alarm indicating a problem with a steam condenser went off and automatically stopped the turbine and the reactor. However, the precise cause of the shutdown was still under investigation.

It said that repair work was being conducted near the condenser when the alarm went off and that workers were investigating whether the maintenance work was linked to the alarm being triggered.

With the suspension, only one of the four reactors was operating at the plant. The remaining two reactors had been shut down previously for regular inspections.

A 9.0-magnitude quake on March 11 triggered a monster tsunami that killed some 20,000 people and crippled cooling systems at the Fukushima Daiichi nuclear power plant, northeast of Tokyo, causing reactor meltdowns.

Subsequent explosions and radiation leaks forced the evacuation of tens of thousands of people from a 20 kilometre (12 mile) radius around the crippled plant, and in some pockets beyond.