Shares in Sistema, which groups the assets of Russian businessman Vladimir Yevtushenkov and includes the country's largest mobile operator MTS, hit a three-year low after Rosneft filed its second lawsuit on Thursday. [L8N1O73P5]
Rosneft is seeking the return of dividends paid out by mid-sized oil company Bashneft in the years between 2009 and 2014, when Sistema was its controlling shareholder.
"Sistema has been forced to take actions to defend the legitimate interests of the corporation, its employees, partners and shareholders, and has asked the court to compensate the losses incurred," Sistema said on Friday.
A spokesman for the group said it had not ruled out further lawsuits to counter the actions of Rosneft and Bashneft, raising the stakes in a long and bitter dispute which has resurrected fears about the risk of doing business in Russia.
The Russian government seized Sistema's stake in Bashneft in 2014, saying its privatisation had been illegal. Rosneft later bought a controlling stake in Bashneft and in May filed its first lawsuit, alleging Sistema had removed assets from the company - something Sistema denies.
A Russian court ruled in August that Sistema should pay Rosneft more than 136 billion roubles in compensation.
Russian news agencies reported Rosneft spokesman Mikhail Leontyev as saying Sistema's legal action was "absurd".
President Vladimir Putin has fuelled speculation about the Kremlin's role in such disputes by calling on Rosneft and Sistema to settle out of court, saying this would benefit both companies and the wider Russian economy.
But the president's intervention appears to have been ignored, with both companies accusing one another of not making compromise proposals.
(Reporting by Denis Pinchuk and Anastasia Teterevleva; writing by Maria Tsvetkova and Denis Pinchuk; Editing by Katya Golubkova and Alexander Smith)