Russia's top oil producer Rosneft (ROSN.MM) has agreed to buy gas assets from diamond miner Alrosa (ALRS.MM) for $1.4 billion, in a drive to expand its natural gas business, Rosneft said on Friday.

Rosneft also said the acquisition of Alrosa assets in the Yamalo-Nenets Autonomous Region and in the Republic of Sakha would increase its recoverable gas reserves by more than 200 billion cubic metres (bcm).

Led by Igor Sechin, Rosneft has ambitions to become Russia's second-largest gas producer with output of 100 bcm of gas a year by 2020, up from about 13 bcm last year, compared with dominant gas producer Gazprom's (GAZP.MM) 479 bcm.