REUTERS Russia&#39;s gas giant Gazprom has stopped invoicing NJSC Naftogaz of Ukraine for natural gas supplied to the occupied areas of Donbas, eastern Ukraine. "They have issued no invoice for January  and the entire 2017," a source on the Ukrainian market told the Ukrainian economic news outlet Ekonomichna Pravda. Read alsoUkraine&#39;s government starts talks with potential partners in managing GTSThe Russian company has not mentioned any cause behind this decision, it said. It could be related to Gazprom&#39;s failure last year in a case against Naftogaz in the Arbitration Institute of the Stockholm Chamber of Commerce, which considered, among other things, payment for gas supplied to the occupied Donbas. Gazprom&#39;s claim for gas supplies to Donbas was estimated at about US$1.3 billion. Russia annually supplied about 2 billion cubic meters of natural gas to areas beyond Ukraine&#39;s control in Donetsk and Luhansk regions. Gazprom unilaterally decided to start supplying gas to eastern Ukraine in 2015.