A bomb and gun attack on a hydroelectric power station in Russia's Kabardino-Balkaria region has killed two guards and injured two others.
Four bombs caused a fire in the Baksan plant's engine room which was later extinguished, prosecutors said.
Supplies to the electricity grid were not affected, according to the state-run RusHydro company
The plant on the Baksan river lies in a part of the North Caucasus gripped by a violent Islamist insurgency.
Officials said the flow of water from the dam had been stopped to prevent any flooding downriver.
Kabardino-Balkaria has seen less militant violence than the nearby regions of Chechnya, Dagestan and Ingushetia.
The most serious attack came in October 2005 when dozens of men stormed the regional capital Nalchik, leaving 136 people dead including 91 militants, according to the Russian government.
RusHydro said in a statement on its website that explosions had hit the plant at 0525 local time (0125 GMT) on Wednesday.
The attackers detonated four explosive devices in the 25-megawatt Baksan plant while a fifth failed to go off, prosecutors said.
According to police spokesman Adlan Kakakuyev, two cars carrying half a dozen assailants had attacked the plant, shooting two guards and wounding three other people.
The attackers reportedly seized two Kalashnikov assault rifles from the dead guards.
The same group are believed to have earlier opened fire on a police station in the town of Baksan.
No group was saying immediately that it had carried out the attack, the agency adds.
The Baksan hydroelectric power station was built in the 1930s.
"There is no danger of a technical accident or disaster," regional official Gennady Vykhristyuk said in televised remarks.
"The power plant's staff are alive and well."
According to Russia's Ria-Novosti news agency, Russian Prime Minister Vladimir Putin has put Deputy Prime Minister Igor Sechin in charge of repairing the damaged power station.
