Ukrainian naval drones hit two sanctioned tankers in the Black Sea as they headed to a Russian port to load up with oil destined for foreign markets, an official said on Saturday, as Kyiv tries to pile pressure on Russia’s vast oil industry.

The two oil tankers, identified as the Kairos and Virat, were empty and sailing to Novorossiysk, a major Russian Black Sea oil terminal, the official at the Security Service of Ukraine told Reuters.

Naval drones could be seen speeding towards hulking tankers followed by powerful explosions that caused fires on the vessels, video footage shared by the official showed.

Reuters could not independently verify the identity of the tankers in the clips or the location and date of the footage.

“Video shows that after being hit, both tankers sustained critical damage and were effectively taken out of service. This will deal a significant blow to Russian oil transportation,” the official said in a written statement.

Ukraine has been attacking Russian oil refineries for months, using long-range aerial drones to strike far behind the front lines of Moscow’s full-scale war against Ukraine. The strikes on the tankers represent a different kind of attack.

Kyiv has repeatedly called on the West to take real action against Russia’s so-called “shadow fleet”, which the Ukrainian government says is helping Moscow export large quantities of oil and fund its war in Ukraine despite Western sanctions.

The fleet of hundreds of often ageing, unregulated vessels came to prominence after Russia’s invasion of Ukraine, bypassing Western sanctions aimed at reducing Moscow’s oil revenue.

Separately, the Caspian Pipeline Consortium, which handles more than 1% of global oil, said on Saturday it halted operations after a mooring at Russia’s Black Sea terminal was significantly damaged by a Ukrainian naval drone attack.

CPC exports mainly from Kazakhstan via Russia and the Black Sea terminal. Kazakhstan called the attack unacceptable