Ukraine investigates ex-PM over Russian gas deal

Tymoshenko, who stepped down as prime minister after losing the February 2010 presidential election to Viktor Yanukovich, has already been targeted by two separate criminal probes related to alleged misuse of state funds, charges she denies.

"A criminal case has been launched against the former prime minister over abuse of power linked to the signing of gas contracts in 2009," First Deputy Prosecutor General Renat Kuzmin told reporters.

He was referring to a January 2009 deal between Tymoshenko and Russian Prime Minister Vladimir Putin on Russian gas supplies, which are crucial for Ukraine's energy-hungry economy.

The agreement ended a stand-off that had briefly disrupted Russian gas flows to Europe. It tied the price of gas from Russia's Gazprom to spot oil prices. As a result, Ukraine's gas bill has since been steadily rising.

Yanukovich's government has urged Russia to review the pricing formula but talks have not been successful so far.

Tymoshenko, who accused the Yanukovich camp of election fraud in 2010, remains his main political rival, though she has failed to marshal other opposition parties behind her.

She says that the criminal cases opened against her are politically motivated and that she and her allies, some of whom have been arrested, are being victimised by Yanukovich.

(Reporting by Pavel Polityuk; Writing by Olzhas Auyezov; Editing by Richard Balmforth and Mark Trevelyan)