KIEV (Reuters) - Ukraine will investigate a suspected cyber attack on its power grid, the energy ministry said on Thursday, an incident the country's secret service has blamed on Russia.

A power company in western Ukraine, Prykarpattyaoblenergo, said on Dec. 23 that a swath of the area it serves had been left without energy, including the regional capital Ivano-Frankivsk, due to "interference" in the work of the system.

Ukraine's SBU state security service blamed Russia, which has not so far commented on the allegation. The energy ministry in Kiev said on Thursday that it had set up a special commission to investigate what happened.

While cyber attacks are commonplace, few successful assaults on industrial targets have been documented. If the SBU's accusations are validated by the probe, it would be the first time a specific power outage has been credibly linked to a cyber attack, said Robert Lee, a former U.S. Air Force cyber warfare operations officer.

Lee said it was too early to say whether the SBU's account was credible. If the claims are validated, then the incident could prompt other nations to use similar tactics.

"Once there is a precedent, that would open up avenues for states to feel comfortable in going that route," said Lee, chief executive of cyber security firm Dragos Security.

The SBU said in a statement on Monday that it had managed to thwart malware that was launched by "Russian security services".

"It was an attempt to interfere in the system, but it was discovered and prevented," an SBU spokeswoman told Reuters on Thursday, adding that the region would have faced a much longer blackout if the malware had executed as the attackers had intended.

The Kremlin could not immediately be reached for comment.

Cyber security experts consider Russia one of the world's most advanced cyber powers, along with the United States, China, Israel, France and Britain.

Relations between Russia and Ukraine have sharply deteriorated since Moscow annexed Crimea last year and supported pro-Russian rebels in eastern Ukraine.

Russia has complained that it itself has become a target, saying Russian security services had detected a sharp rise in cyber attacks after the Ukraine crisis worsened and ties with the West deteriorated.

Crimea has lost at least one quarter of its power after Ukraine switched off supplies to the contested peninsula on Wednesday, a situation that Ukrainian police blamed on unidentified saboteurs blowing up an electricity pylon.

(Additional reporting by Jim Finkle in Boston; Editing by Andrew Osborn and Richard Balmforth)