Russia’s oil pipeline monopoly Trasneft warned producers that it may be forced to reduce output amid continued Ukrainian drone attacks on seaport and refinery infrastructure, Reuters reported on Tuesday, citing industry sources familiar with the matter.

Ukraine has ramped up its attacks on Russia’s oil infrastructure in recent months, targeting a key source of revenue for the Kremlin’s war effort.

According to sources cited by Reuters, Ukrainian drones have hit at least 10 refineries since the beginning of August, which led to a reduction in refining capacity by nearly 20% at one point.

Last week, drones struck Primorsk, Russia’s largest oil port northwest of St. Petersburg, temporarily halting operations.

In response to the ongoing attacks, Transneft has limited producers’ ability to store oil in its pipeline system and warned that further damage could force the company to accept less oil from producers, two industry sources told Reuters.

Transneft called the report “fake” and part of the West’s “information war” against Russia, which accounts for 9% of global oil production. Russian authorities have not publicly commented on the extent of damage to oil infrastructure.