* Plant owner Rushydro says plant shut by fire on Tuesday
* Unofficial Islamist website says two bombs exploded
* No reports of casualties
MOSCOW, Sept 8 (Reuters) - An Islamist website has reported that bombs planted by rebels exploded at a hydro electric power station in Russia's southern Dagestan region that was closed this week because of a fire.
The unofficial website www.kavkazcenter.com said two bombs had detonated at the Irganaisky plant while a third had failed to explode.
Russian news agencies, citing a local law enforcement source, reported on Thursday that one bomb with the equivalent of at least 3 kilogrammes of TNT had been discovered at the plant and defused.
There were no reports of any serious injuries related to the incident.
The owner of the plant, Rushydro <HYDR.MM>, said in a statement on Wednesday that the power station had been closed due to a fire that broke out on Tuesday in the turbine room.
But company spokeswoman Yelena Vyshnekova said she could not say what caused the fire, referring Reuters to several Russian media reports that suggested the fire was an industrial accident.
"We cannot comment on the cause until the investigation ends. But state investigators and the police have said they believe it was not a terrorist act," she said.
Rushydro said the electricity supply to the region had not been affected by the closure of the 400 megawatt plant.
Russia is struggling to contain an upsurge of attacks in the mainly Muslim provinces along its southern flank, including Dagestan, Chechnya and Ingushetia. Rebels, who in March took their war to the Russian heartland with deadly bombings in the Moscow metro, have promised to focus on economic targets.
The website is the main media outlet used by the rebels, but it has published several implausible claims of responsiblity in the past.
Observers widely dismissed the site's claim that rebels were responsible for the Sayano-Shushenskaya dam disaster in Siberia last year that killed 75 people.
Local leaders say a potent mix of clan feuds, poverty, Islamic extremism and heavy-handed tactics by law enforcement agencies has driven youths into the hands of rebels who want to create a Sharia-based pan-Caucasus state.
In July, militants stormed and bombed a hydro-electric power station in the Muslim republic of Kabardino-Balkaria, putting it out of action for two years.
(Reporting by Conor Humphries; Editing by Noah Barkin) (conor.humphries@thomsonreuters.com; +7495-7751242)
Our Standards: The Thomson Reuters Trust Principles.
