A traffic jam of oil tankers has grown off Turkey after the imposition of a price cap on Russian crude by western powers attempting to hurt the Kremlin’s coffers, report The Guardian.

The vessels have come to a halt after Turkish authorities in Ankara demanded that insurers prove that the ships heading through its straits are fully insured.

EU sanctions on Russian oil prices came into force on December 5, after tense negotiations the previous week. The rules state that tankers carrying Russian crude oil must not carry western maritime insurance unless it is sold under the $60 a barrel G7 price cap.

The cap has been introduced in an attempt to curb Russia’s fossil fuel revenues while ensuring oil continues to flow and the maritime insurance industry, which is dominated by companies in London, was not damaged.

Russian oil transferred via a pipeline is not covered by the cap.

About 19 crude oil tankers were waiting to cross Turkish waters on December 5, stopping near the Bosphorus and Dardanelles, which link Russia’s Black Sea ports with overseas markets, the Financial Times reported. It said one tanker had been waiting for six days.

The tanker buildup is the first sign of the impact of the cap on the international oil market, which has been upended by the war in Ukraine.