UPDATE 1-Sistema says Russian court "arrests" some of its holdings in Rosneft dispute

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MOSCOW, June 26 (Reuters) - Russian conglomerate Sistema said on Monday a Russian court had "arrested" shares it owns, including in the country's biggest mobile operator MTS, in a legal dispute with oil company Rosneft.

Rosneft is suing Sistema for 170.6 billion roubles ($2.9 billion) in damages over the purchase of oil producer Bashneft . Sistema proposed an out-of-court settlement last week.

But on Monday, Sistema said it had received notice of an enforcement action from the Moscow Directorate of the Federal Bailiffs Service and a copy of a court order from the Republic of Bashkortostan Arbitration Court.

According to the court order, an arrest has been imposed on a 31.76 percent stake in MTS, 100 percent of its Medsi chain of medical clinics, and 90.47 percent of Bashkirian Power Grid company owned by Sistema and Sistema-Invest.

"The arrest of shares was made following a decision by the Republic of Bashkortostan Arbitration Court as a security in the framework of the legal claim lodged by Rosneft, Bashneft and the Republic of Bashkortostan against Sistema and Sistema-Invest," Sistema said on Monday.

Rosneft spokesman Mikhail Leontyev told Reuters that arrest of the shares was a "security measure" and the shares arrested equalled value of Rosneft claims against Sistema.

"Shares are not being seized but a number of actions such as sale, purchase, usage as a collateral, are being suspended ... Rosneft needs a fair compensation of its claims and these assets act as guarantee for this becoming possible," Leontyev said.

Sistema said in the statement that its considers demands "unlawful and unfounded." It said that the arrest limits its and Sistema-Invest's rights to receive any income on the shares in question.

"This situation does not impact operations with MTS shares, and the rights to receive dividends on MTS shares owned by other shareholders," MTS said in a separate statement.

($1 = 58.9317 roubles) (Reporting by Jack Stubbs, Anastasia Teterevleva and Vladimir Soldatkin; writing by Katya Golubkova; editing by Jane Merriman)