A small fire in a turbine hall shut down a Swedish nuclear reactor overnight but the blaze was swiftly extinguished, nuclear power plant officials said Sunday.

"A fire alarm went off around midnight and our own firefighters were able to locate the fire in the turbine hall and quickly extinguish it," Anders Oesterberg, a spokesman at the Oskarshamn nuclear power plant, told AFP.

The cause of the fire was not immediately known, but an oil leak in the turbine was seen as a possible cause, Oesterberg said.

"A fire at a nuclear power plant is always serious, but this was a relatively minor incident," Oesterberg said.

The Oskarshamn reactor number two remained closed Sunday at midday and it was not yet known when it would start up again.

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.