“Greetings from the Birds of the Unmanned Systems Forces,” said Ukraine’s drone leader in August, Robert Brovdi, after attacks on Russian oil infrastructure.

Over the past two months, Ukraine has intensified its campaign against its enemy’s oil refineries, hitting 21 of its 38 large plants. On Friday, a drone struck the Orsk refinery on the Kazakh border more than 1,500km from Ukraine.

Why is Ukraine launching such attacks? Will they seriously hamper Russia’s war economy or its ability to fight? And what might Kyiv do next?

Stalemate on the front lines in eastern Ukraine has led both sides to seek other ways to wage war. Russia, for its part, has conducted its largest drone and missile assault ever on Ukraine’s gas sites on Friday, trying to cut off power and heating as winter approaches.

About 80 per cent of Ukraine’s long-range weapon strikes on Russia have attacked petroleum sites. Kyiv has clearly identified this as its adversary’s key vulnerability.

Attacking refineries is one way to harm Russia’s economy. They are large, vulnerable, explosive targets, but are usually away from urban areas, thus minimising civilian casualties. This is in sharp contrast to Russia’s incessant attacks on Ukrainian cities. They supply fuel to Russia’s armed forces, so they are arguably a legitimate military target. Blowing up refineries does not directly stop Russian oil flows, which could cause world prices to rise sharply and diminish European and American support.

Refineries not only supply Russia’s domestic market, but also account for a large part of its oil exports. It has only three main customers for its crude now, China, India and Turkey. In contrast, exports of products such as diesel and petrol can be marketed much more widely, in smaller quantities, and blended or laundered with other countries’ products to disguise the origin.

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Ukraine waged a similar drone campaign starting in April last year. That caused damage but no serious or lengthy interruptions to Russian refining or fuel supply.