A nuclear reactor in Sweden stopped automatically on Wednesday after an alert went off, but the cause had yet to be determined, its operator OKG said.

"Shortly after midnight on September 26 there was an automatic and rapid halt at Oskarshamn 3. The reason for the halt was an unexpected signal in the reactor's security system," OKG said in a statement.

"In order to be able to determine the cause of the halt to production... work to analyze the incident has been launched," it added. But no problem was detected on Wednesday.

OKG said it expected to be able to restart the reactor on Saturday.

Oskarhamn, on the east coast of Sweden, around 300 kilometers (180 miles) south of Stockholm, is one of Sweden's three nuclear power plants.