"An increased leak in the primary system within the containment building was detected," officials at the Krsko plant, around 100 kilometres (60 miles) east of the capital Ljubljana, said in a statement sent to AFP.

It said the leak had no repercussions on employees or surrounding residents and the environment.

"The shutdown is needed to determine the exact source of the leak and the measures needed to fix it," the statement added.

The last precautionary shutdown at Krsko occurred in 2020, after an earthquake that shook neighbouring Croatia, killing seven people.

In service since 1983, the 700-megawatt reactor at the plant provides about 20 percent of Slovenia's electricity needs and about 15 percent of the needs in Croatia, which co-owns the plant.

Originally due to be switched off in 2023, the two countries agreed to extend its lifetime by 30 years.

Slovenian authorities are also considering the construction of a second reactor at the site, despite concerns over its ageing structures and the risks from seismic activity in the region.