Russian services conglomerate Sistema said on Thursday it had bought an additional 21 percent stake in India's Shyam Telelink, taking its holding in the telecom operator to 72 percent from 51 percent.

Sistema, majority owned by billionaire Vladimir Yevtushenkov, acquired a 51 percent stake in Shyam Telelink earlier this year and entered into a call option agreement which gave it the right to increase its stake to a maximum of 74 percent within approximately 16 months.

On May 23 Sistema signed an amendment to the call option agreement which allowed it to speed up the purchase of an additional 21 percent stake in the Indian firm for $45 million, Reuters reported.