With its army short of ammunition and troops to break the deadlock on the battlefield, Ukraine has increasingly taken the fight behind Russian lines, attacking warships, railways and airfields in an attempt to diminish Moscow’s military operations.

Most recently, that campaign has focused on oil infrastructure, hitting refineries deep in Russian territory and driving home the country’s vulnerability to such attacks.

On Tuesday and Wednesday, Ukrainian drones hit four Russian refineries, officials on both sides said, adding to a series of recent attacks that have set fire to depots, fuel tanks and other oil infrastructure across Russia. Since the beginning of the year, Ukraine has claimed responsibility for nearly a dozen such assaults, and local Russian authorities have reported five more.