Russia’s energy sector is struggling in the face of Western sanctions to repair their refineries, built with the help of U.S. and European engineering firms, according to at least 10 Russian industry sources.

The difficulties have been exacerbated by Ukrainian drone attacks that have struck at least a dozen Russian refineries this year, the industry sources said. The attacks forced Russian refineries to shut in some 14% of capacity in the first quarter, according to Reuters calculations.

“If the stream of drones continues at this rate and Russian air defences don’t improve, Ukraine will be able to cut Russian refining runs quicker than Russian firms will be able to repair them,” said Sergey Vakulenko, an expert on Russia’s energy industry and non-resident fellow at the Carnegie Endowment for International Peace, an international affairs think tank.