Kiev has so far recognised the price of $268.5 only for gas that had already been supplied. Ukraine's state-owned energy firm, Naftogaz, has filed a lawsuit at the international arbitration court in Stockholm to establish a "fair and market price" for natural gas supplies from Gazprom. Tensions are high between Moscow and Kiev over a separatist rebellion in eastern Ukraine, in which the Ukrainian authorities accuse Russia of directly backing the armed rebel fighters, and over Russia's annexation of the Crimea peninsula in March. Moscow denies supporting the rebels militarily and is wary of Kiev's moves to deepen ties with the EU. Russia's relations with the EU are also strained because of sanctions imposed on it over its role in the Ukraine crisis. EU regulators are also investigating Gazprom for anti-competitive behaviour, including in relation to gas pricing.