The U.S. Treasury Department has sent notices to 30 ship management companies requesting information for more than 100 vessels it suspects of transporting Russian crude oil above the Western oil price cap, according to a source who has seen the documents.

The notices, sent by the Office of Foreign Assets Control to ship management companies in about 30 countries on Friday, represent the biggest step of its kind by the United States since Washington and its allies imposed the price cap last year aimed at restricting oil revenues to Moscow as punishment for its invasion of Ukraine, the source said. The requests for information are a routine step in sanctions investigations.

The Treasury Department did not immediately respond to a request for comment.

The Group of Seven rich countries, the European Union and Australia imposed the $60 per barrel cap last December on sea-borne exports of Russian crude to punish Moscow for its war on Ukraine. It bans Western companies from providing services such as transportation, insurance and financing for the oil sold above the cap.

The price cap has caused a shift in global markets as China and India purchase Russian oil, much of which had traditionally gone to Europe and other markets. A rally in global oil prices this year has meant much of Russian oil has traded above the cap.

U.S. officials have said the cap has imposed extra costs on Russia for a “ghost fleet” of aging tankers, longer voyages and reliance on non-Western maritime services that have cut into the revenues it can spend on the war.

Source: Reuters reported by Timothy Gardner and editing by Richard Valdmanis and Grant McCool