Russia's state-owned oil giant Rosneft has filed a lawsuit against Japanese and other stakeholders in the Sakhalin-1 offshore oil and gas project, demanding some 89 billion rubles ($1.4 billion) for alleged "unfounded" gains, the Tass news agency said Monday.

The claim was filed against five entities, including Sakhalin Oil and Gas Development Co., in which Japanese trading firms Marubeni Corp. and Itochu Corp. own stakes and the Ministry of Economy, Trade and Industry also invests, according to the complaint filed July 20 at a court in the Sakhalin region.

The claims against Sakhalin Oil and Gas Development total some 26.5 billion rubles.

The suit seeks to recover funds gained by the five parties between July 10, 2015, and May 31 of this year through "unjust enrichment and interest gained by using other people's money," according to paperwork issued by the court.

The claim was also filed against U.S.-based Exxon Mobil Corp.

The Sakhalin-1 project, launched in 1995, is a large-scale crude oil and natural gas development project off Sakhalin Island in the Russian Far East, of which 30 percent is owned by Sakhalin Oil and Gas Development. It started oil production in October 2005 and began shipping some of it to Japan in 2006.

"Sakhalin-1 is an important project. We would like to follow the discussions between the related parties while seeking to grasp the situation," Chief Cabinet Secretary Yoshihide Suga told a news conference in Tokyo.

The legal action by Rosneft, led by CEO Igor Sechin, a close aide of Russian President Vladimir Putin, comes as Japan and Russia are deepening economic cooperation, including in natural resources development.