Scores of people missing in an accident at hydro-electric dam are feared dead.
Russian investigators have rejected a claim purportedly made by the Riyadus Salikhiin, a Chechen armed group, that it caused the explosion.
A statement on a website used by the representatives of Islamist groups said the blast had been caused by an anti-tank grenade placed in the turbine hall, but the investigators said there was no sign that it was a “terrorist act”.
The cause of the accident was still unclear, but officials said a faulty turbine and increased pressure in the pipes could have triggered the blast.
The purported Riyadus Salikhiin claim posted on the website said that the group had begun an “economic war” against the Russian government and set up groups “with the aim of carrying out acts of industrial sabotage”.
The group is believed to seek to end Russian rule in the North Caucasus and the establishment of an Islamic emirate in its place.
It is designated as a banned terrorist group by the US state department and in June claimed responsibility for a bomb attack that badly injured Yunus-Bek Yevkurov, the president of Ingushetia.
Analysts were skeptical about the message, which came amid a wave of attacks in the Caucasus region.
“In the case of terrorist attacks, notification about who has carried out an attack usually happens at the time of the attack,” Yulia Latynina, a journalist who specialises in the North Caucasus, said.
“The dam accident happened on Monday morning. If this website had placed the announcement half-an-hour before that, their claim would have been plausible.”
Alexei Malashenko, a Caucasus expert at the Moscow Carnegie Centre, described the claim as “propaganda”.
He said Chechen rebels had in the past taken responsibility for events generally accepted as accidents, including a fire in Moscow’s Ostankino television tower and a mass blackout in the capital.
The power station has been shut down since the powerful explosion that blew out walls and caused the turbine room at Russia’s largest power plant to flood.
Repairs are estimated to take from two to four years, and the energy minister said it would cost $1.2bn to rebuild the turbine room.