* Small amount of explosive found on way into Swedish nuclear plant
* Police say explosive could not have caused much damage, open probe
* Alert levels raised after fresh nuclear safety incident (Adds details, quotes)
STOCKHOLM, June 21 (Reuters) - Sweden raised alert levels at its nuclear power plants and police launched an investigation after a routine check found a forklift truck entering on of the plans with a small amount of explosives on it
Police said on Thursday the small amount of explosive could not have caused much damage had it been set off, but opened a probe into sabotage, without saying who would want to carry out such an act. A spokesman said that so far there were no signs of a terror attempt on the facility.
The incident happened on Wednesday afternoon at the Ringhals nuclear power station, which is on the southwest coast near the city of Gothenburg, operator Vattenfall said in a statement.
The nuclear safety authority said in a statement that Ringhals and the other two active nuclear plants at Oskarshamn and Forsmark had raised their security levels by one step from the lowest level in a four step scale.
Ringhals is one of three active nuclear plants in Sweden, where several safety scares in recent years have increased criticism of nuclear power
" A suspected explosive material was discovered in a truck on its way into the premises of the nuclear power plant Ringhals," state-owned Vattenfall said in a statement.
Police later confirmed that the substance was indeed an explosive material, it said. "There was no detonation device connected to the material. The object could not have caused serious damage at Ringhals," the company added.
Police during the night searched Ringhals with bomb searching dogs and did not find anything else, it added.
Police spokesman Tommy Nyman told Reuters that a small amount of explosive had been found on the vehicle, which was a fork lift truck. He said the substance had been partly concealed. The driver had been questioned but said she did not know the material was on board, he added.
He said the explosive could not have disrupted power production at the plant.
Another spokesman, Ingmar Nilja, said police "have no indications in the direction of terrorism" but declined to say who could have wanted to carry out any act of sabotage.
Sweden's nuclear safety has been criticised in recent years.
In 2006, a short circuit at Forsmark caused one of the reactors to shut down automatically.
In 2010, activists from Greenpeace also broke into Forsmark while in 2011 a fire broke out in a turbine hall at Oskarshamn.
In 1980, Swedes voted in a referendum to phase out existing reactors by 2010, but the centre-right government has granted permission to extend the lifetime of the current plants.
Sweden has 10 active nuclear reactors, which produce about 40 percent of power needs.
Vattenfall has a 70 percent stake in Ringhals and Germany's owns nearly 30 percent.
Editing by Bill Trott/Jeremy Gaunt