Maintenance work triggered an alarm that shut down a fifth Belgian nuclear reactor on Thursday, leaving only two working in the country but posing no danger to the public, the operator said.

The last of the three reactors working at the Tihange plant in southeast Belgium shut down automatically around 1:00 am (2300 GMT Wednesday) following the alarm, said Electrabel, a subsidiary of the French firm Engie.

At Belgium's only other nuclear plant, located in the northern region of Doel, two of the four reactors remain shut down following previous problems.

Pre-planned maintenance work in the Tihange 3 reactor block "triggered an alarm which caused activity to stop," Electrabel spokesman Serge Dauby told AFP.

"We are still in the process of analysing the reasons it stopped," company spokeswoman Geetha Keyaert added. "There was never any danger for the surroundings, the employees or the plant."

Dauby said that "from the middle of the night, technicians and experts were called in to study the causes and needs for repairs and maintenance so that we can come back on line as soon as possible."

Electrabel said in a statement later that it aimed to restart the reactor overnight Friday.

The difficulties besetting Belgium's nuclear plants have hurt financial results at Engie, whose profits fell 56 percent to 1.11 billion euros in the first half of the year.

The Doel 3 and Tihange 2 reactors were shut down in March 2014 to allow for additional tests on reactor vessels where thousands of micro-cracks were discovered in 2012.

They are due to go back on line on November 1 at the earliest.

Engie said in May that closing down the Doel 3 and Tihange 2 reactors were costing the company around 40 million euros a month in net profit.