The U.K. government has Increased its efforts to clamp down on the Russian so-called shadow fleet of tankers that are being used to export oil and fund Vladimir Putin’s war on Ukraine. The Guardian reports that the U.K. Foreign Office announced September 10 it was imposing sanctions on ten ships it believes to be at heart of the operation.

Russia uses a large fleet of often unseaworthy and ageing tankers to transport Russia’s gas and oil that are its most critical revenue source for funding the war in Ukraine, accounting for about a quarter of the Russian budget in 2023.

Speaking on his way to Kyiv to meet Ukrainian leaders, the U.K. foreign secretary, David Lammy, said: “Putin’s war machine is funded by a dark and illicit economic system that this government is committed to destabilizing.”

It is the third time the U.K. has attempted to crack down on the shadow fleet, bringing the total number of ships sanctioned to 25.

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The ships targeted are all described as “high-volume offenders” — vessels operating around the clock to transport as much Russian oil as possible. These ships will now be barred from entering U.K. ports and will be refused access to the U.K. ship register.

In an effort to cut off revenue, but at the same time prevent the price of oil skyrocketing, the G7 nations (Canada, France, Germany, Italy, Japan, the U.K. and the U.S., with the EU participating as a “non-enumerated” member) agreed to block insurance to any ship that sold oil at above a cap of $60 a barrel. The shadow fleet has grown to circumvent the G7 policy, but analysts have said a relatively small number of ships are at the heart of the operation.