A report shows how Russia has largely evaded sanctions aimed at limiting its revenue from oil sales.

A plan hatched by wealthy Western nations to deprive Russia of oil revenue is largely faltering, a new report found, with the majority of the Kremlin’s seaborne oil exports evading restrictions that were supposed to limit the price of Russian crude.

Almost two years since an oil “price cap” was enacted, nearly 70 percent of the Kremlin’s oil is being transported on “shadow tankers” that are evading the restrictions, according to an analysis published Monday by the Kyiv School of Economics Institute, a Ukraine-based think tank.

Russia’s success at circumventing the sanctions imposed by the Group of 7 nations has allowed it to continue to finance its war against Ukraine. The effectiveness of the price cap has been marred by loose enforcement of the policy. Officials in the United States and Europe have tried to balance their goals of crippling Russia’s economy while keeping oil markets well supplied to prevent price spikes.

The challenges underscore the limitations that the world’s advanced economies have been facing as they attempt to intervene in global energy markets to try to hasten an end to Russia’s invasion of Ukraine.

The Kyiv School of Economics Institute, which has argued for tougher sanctions on Russian oil, noted in its report that Russia’s shadow fleet poses a threat to the world’s oceans because the tankers are often poorly maintained and not properly insured.

“There have been several instances of shadow tankers being involved in collisions or coming close to running aground in recent months,” the report said. “Large oil spills have so far been avoided but a major disaster is waiting to happen and cleanup costs would reach billions.”

After Russia invaded Ukraine in 2022, the United States and Europe moved to ban imports of Russian oil, in an effort to reduce revenues for one of the world’s largest oil producers.

But amid concerns that a full embargo could cause a global oil price shock, the United States and other wealthy democracies settled on an alternative plan. They decided to use the maritime industry, including the shipping companies and insurance carriers that had been the primary conduit for transporting Russia’s seaborne oil, to ensure that any oil traveling by sea could be sold only at a discount: $60 per barrel, which was about $15 a barrel less than the price on the global market.

But Russia soon found a workaround. It has invested about $10 billion in developing its fleet of shadow tankers, which are often unmarked, allowing it to sell much of its oil above the cap level. According to the Kyiv institute, in the first half of this year more than 75 million barrels of Russian oil every month were transported on ships that are on average 18 years old.

Energy analysts have also observed that ship operators have been willing to flout the sanctions by registering their vessels in countries that are not part of the price cap agreement, or by changing their ownership structure.

A report by Lloyd’s List Intelligence this month found that record volumes of Russian oil products were carried in September on tankers that are part of its shadow fleet or that have already been sanctioned for violating the price cap. It noted that some of those ships are bringing cargo to China and India, which apparently are willing to disregard Western sanctions.

The rise of the Russian shadow fleet has stirred debate within the Biden administration this year over how aggressively to enforce the price cap. Some Treasury officials have urged a tougher approach, which could include more sanctions on shadow fleet ships, while other White House economic advisers have cautioned against taking steps that could disrupt oil supplies and raise gasoline prices ahead of the presidential election.

Policymakers have been particularly cautious in their approach to Russia’s oil sector as conflicts in the Middle East have been escalating. Oil prices have been hovering around $75 per barrel, but analysts have warned that they could jump if a war breaks out between Israel and Iran.

Washington has imposed sanctions on ships that are part of Russia’s shadow fleet, and Western allies such as Britain and the European Union have been taking steps to crack down on the vessels in recent months. Following a meeting of sanctions officials from the European Union, Britain and the United States last month, the European Commission said that enforcement of the price cap remained a priority for fulfilling its goal of “squeezing Russian oil revenues used to finance its illegal war.”

A U.S. official who requested anonymity to discuss the Biden administration’s thinking said that it was not surprising that President Vladimir V. Putin of Russia has found ways to evade the sanctions over time and that the “price cap coalition” is continuing to work with the shipping and finance industries to enforce the policy. In addition, the official said, the fact that Russian oil prices remain depressed relative to oil sold by producers from other countries, and that Mr. Putin has had to spend billions of dollars to circumvent the cap, has made the policy a success.

The Kyiv institute is pushing for Western countries to crack down on Russia’s sanctions evasion by creating “shadow free” zones.

This would require ships to have more extensive oil spill insurance that could be verified, expand the use of sanctions for violations and in some cases give nations in the coalition the authority to stop shadow fleet ships that are not properly insured in international waters.