Sweden has tightened security at its three nuclear power plants after routine checks revealed an explosive device planted on a truck inside its southern Ringhals plant, police said Thursday.

"Preliminary results from the forensic analysis confirm that it was explosive material. Exactly which type it was and the exact quantity is not something we want to disclose at this point," police said in a statement.

The explosives, discovered Wednesday during a routine check, "could not have caused any major damage", the Ringhals plant said in a separate statement.

"There was no risk of explosion since the explosives did not have a detonator," police explained.

The material resembled a dough-like lump the size of a small fist and was discovered by a bomb-sniffing dog under a large truck in a secure zone. The discovery was made during a routine check as the truck passed between two inner zones, Ringhals spokesman Goesta Larsen told Swedish news agency TT.

The object was stuck to a fire extinguisher placed under a step leading to the truck driver's cab.

The Ringhals plant, located in southern Sweden, is the country's biggest nuclear plant with four reactors, producing about 20 percent of Sweden's electricity.

Police said they have opened a preliminary inquiry into suspected sabotage, adding that no arrests had been made. The truck driver was not suspected of any wrongdoing.

The truck had been in service at the plant and had not left the area "for a long time," Larsen told TT.

Bomb technicians searched the Ringhals plant but no other suspect objects were found.

The Swedish Radiation Safety Authority (SSM), in charge of nuclear safety, said Ringhals had raised its security level by one notch, to the second-lowest level, as a precaution "until more is known".

"Ringhals has also informed the other nuclear plants about the incident, and they have also raised their security levels," SSM said in a statement.

The authority had placed Ringhals under observation in 2009 after a series of incidents observed since 2005 that could endanger security, including weaknesses in management and governance, lack of traceability of internal decisions, and failure to adhere to routines and instructions.

Sweden has 10 nuclear reactors at three plants and the country's parliament passed a landmark bill in June 2010 allowing the reactors to be replaced at the end of their life spans instead of simply ending nuclear power when they expire.