KYIV, Jan 4 (Reuters) - The volume of Russian gas transported via Ukraine to Europe fell 25% last year, reducing a key revenue stream for Ukraine"s struggling economy at a time of heightened tensions with Russia. The state operator of Ukraine"s gas transmission system said on Tuesday that 41.6 billion cubic metres (bcm) of Russian gas was transported through Ukraine to Europe in 2021, equivalent to a daily average of 114 million cubic metres (mcm). That was, however, above the 40 bcm that Russia"s Gazprom had pledged to ship via Ukraine last year. Relations between Kyiv and Moscow plummeted after Russia"s annexation of Crimea in 2014 and the outbreak of war in eastern Ukraine. Moscow has also been developing other routes for transporting gas to Europe, threatening a revenue stream for Ukraine that totalled $2 billion in 2020. Kyiv has accused Russia of cutting gas supplies via Ukraine to drive up prices and put pressure on the European Union to certify another pipeline - Nord Stream 2, which is ready to pump Russian gas to Europe bypassing Ukraine. Earlier on Tuesday, the Ukrainian operator said Gazprom had cut its daily gas supply to Europe via Ukraine to about 50 mcm, the lowest level since January 2020 and giving a fresh boost to prices. read more Russia has said it meets all its contractual obligations on gas exports and that EU policies are to blame for the surge in prices. read more