Swedish nuclear facilities were placed on alert Thursday after explosives were found at the Ringhals nuclear power plant, the Radiation Safety Authority said.

Authority spokesperson Maria Strahle told the Swedish news agency TT the threat level was raised as a precaution following the incident at the plant south of Gothenburg.

An explosive device was found Wednesday afternoon on a large truck that was returning to the reactor area from an industrial estate belonging to the nuclear power plant.

"Under the first step onto the truck there is a fire extinguisher and that is where the explosive had been placed," said Gosta Larsen of the Ringhals plant.

Larsen told TT the device was the size of a small fist.

Police have no suspects but truck drivers and officers are being questioned, officials said. The explosive device was sent to Sweden's National Laboratory of Forensic Science for testing.

The incident has been classified as suspected sabotage.