MOSCOW, April 27 (Reuters) - Russian oil exporters may suspend supplies to Belarus’s Naftan refinery after the United States tightened sanctions on Belarus over alleged human rights violations and abuses, four sources familiar with the matter told Reuters. Washington last week revoked authorisations for certain U.S. transactions with nine sanctioned Belarusian state-owned enterprises, including Naftan and its owner Belneftekhim. While not directly affected by the move, Russian companies are concerned they could be penalised if they continue dealing with the Belarusian businesses. Apart from loans, Russia supports its neighbour and fellow former member of the Soviet Union with oil supplies on beneficial terms. But that may change after the recent U.S. order, the sources said, speaking anonymously due to the sensitivity of the matter. Russia’s Rosneft and Surgutneftegaz do not plan to supply oil to Naftan in May, two of the sources said. Naftan is one of Belarus’s two refineries and has a processing capacity of around 200,000 barrels per day. Russia’s Tatneft, Russneft and Neftisa also supply oil to Naftan. “We are waiting for our bosses’ decision, not signing off (orders) for now,” one Russian oil company source said. Russia supplies Naftan with 5.5 million barrels of oil per month and the four sources said they now expected the volumes to be redirected to sea ports for export. Rosneft, Surgutneftegaz, Tatneft, Russneft and Neftisa did not immediately reply to Reuters’ requests for comment. Belneftekhim declined to comment. Belarus president Alexander Lukashenko, in power since 1994, and a number of his allies were added to U.S. Treasury’s Office of Foreign Assets Control list of Specially Designated Nationals in 2006 following presidential elections that year which Washington said were not free and fair. The sanctions were later expanded to cover Belneftekhim, the Naftan refinery and other Belarusian state firms, but with certain wavers. The wavers expire on June 3, 2021. (additional reporting by Natalia Chumakova and Ludmila Zaramenskikh. Writing by Katya Golubkova. Editing by Mark Potter)