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A Russian-chartered oil tanker in the sea off Morocco in an area identified by maritime technology company Windward as a hub for smuggling oil.

"A cap enforcement crackdown runs the risk of driving up crude prices," he said. "Plus, using 'secondary' sanctions to enforce the cap could push reputable insurers out of the Russian crude game entirely, leaving the market to potentially insolvent stand-ins."

Kevin Book, managing director of research at Clearview Energy Partners, told CNBC that despite the G-7's June and September calls for improving the price cap, and recent guidance urging parties to Russian petroleum transactions to better scrutinize cargoes, "a U.S. pinch on Russian petroleum seems unlikely until after the election."

In a recent report to clients, Clearview Energy Partners characterized the G-7 price cap on Russian petroleum exports to third countries as "increasingly loose."

Ukraine allies, including the U.S., have banned the import of Russian crude, while a price cap imposed on Russian oil by the G7 countries, the European Union and Australia bans the use of Western maritime services such as insurance, flagging and transportation when tankers carry Russian oil priced at or above $60 a barrel to nations where a ban is not enforced.

According to Clearview Energy Partners, Russian crude prices over the last four weeks have averaged about six cents below the Brent crude price. That is far off the trading discount when the cap was first put in place. When the cap was fully phased in, in February 2023, Russian crude was selling at a 30% discount. A year ago, the discount was about 16%.

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