HOUSTON — The United States and the European Union have been unwilling to put sanctions on Russian energy exports in response to the country’s invasion of Ukraine. But some oil traders appear to have concluded that buying oil from Russia is just not worth the trouble.

One of the three top oil producers in the world, after the United States and Saudi Arabia, Russia provides roughly 10 percent of the global supply. But in recent days traders and European refineries have greatly reduced their purchases of Russian oil. Some have stopped altogether.

Buyers are pulling back because they or the shipping companies, banks and insurance companies they use are worried about running afoul of Western sanctions in place now or those that might come later, energy experts said. Others are worried that shipments could be hit by missiles, and some just don’t want to risk being seen as bankrolling the government of President Vladimir V. Putin.

Russian exporters have been offering the country’s highest-quality oil at a discount of up to $20 a barrel in recent days but have found few buyers, analysts said. Buyers, in Europe in particular, have been switching to Middle Eastern oil, a decision that has helped drive the global oil price above $100 a barrel for the first time since 2014.

“The enablers of oil exports — the banks, insurance companies, tanker companies and even multinational oil companies — have enacted what amounts to a de facto ban,” said Tom Kloza, global head of energy analysis at the Oil Price Information Service. Mr. Kloza said it could take weeks before it was clear how significantly Russia’s oil exports had fallen and whether the drop would be sustained, but “clearly the Russian contribution to world oil supply has been constricted.”

On Tuesday, the International Energy Agency said its members, which include the United States and more than a dozen European nations, had agreed to release 60 million barrels of oil from their strategic reserves. The announcement had little impact on global oil prices, probably because the amount was modest, amounting to roughly three days of consumption by the United States. The White House and Energy Department signaled that more oil could be released later by describing the I.E.A. agreement as an “initial release.”

Much of Russia’s oil is shipped out of Black Sea ports for use in Europe. Some shipping companies carrying oil and commercial goods are afraid that their vessels will be fired on. Congestion in sea lanes is interrupting the shipping of not only oil but also food. On Friday, an unidentified missile hit a Moldovan-flagged tanker carrying oil and diesel.

“Russia’s flagship Urals blend was one of the first to break through the $100-per-barrel mark this year,” said Louise Dickson, senior oil market analyst at Rystad Energy, a research and consulting firm. “But the country’s incursion into Ukraine has now made it one of the most toxic barrels on the market.”

As European refiners buy more oil from places like Saudi Arabia, Russian companies are increasingly trying to sell their crude to refineries in China and other Asian countries by offering them discounts.

Most of Russia’s roughly five million barrels of daily oil exports go to Europe. About 700,000 barrels a day are consumed in the United States, roughly 4 percent of the U.S. market.

Several Scandinavian refiners, including Neste Oyj of Finland and Preem of Sweden, have said they halted purchases of Russian oil.

“Due to the current situation and uncertainty in the market, Neste has mostly replaced Russian crude oil with other crudes, such as North Sea oil,” said Theodore Rolfvondenbaumen, a Neste spokesman. As the company watches future sanctions and “potential countersanctions,” he said, it is preparing “for various options in procurement, production and logistics.”

Energy experts say the international oil trade could be rejiggered in ways that are similar to what happened in 1956 when Britain, France and Israel attacked Egypt and closed the Suez Canal. For a time, oil tankers were rerouted around Africa. Similarly, over the next few months Russian oil once shipped to Europe could go to China.

“The trade-off could take six to eight weeks,” said Michael Lynch, president of Strategic Energy and Economic Research, who is an occasional adviser to the Organization of the Petroleum Exporting Countries. “For a week or two, things could be unsettled.”

Mr. Lynch said the decision by European buyers to move away from Russian oil would give China and its president, Xi Jinping, more sway and influence in the energy market and with Mr. Putin.

“If China wants to, they can use the power of their purse to either humiliate Putin by denying him a customer or to elevate him by bailing him out financially,” he said.

While Western countries have placed tough sanctions on Russian banks and wealthy allies of Mr. Putin, they have left the Russian oil industry alone so as not to interrupt global supplies and further fuel inflation.

As the fighting in Ukraine has escalated this week, and hopes for a negotiated settlement has faded, global and American oil prices have jumped. The market was tight even before the crisis, because demand has been rising as coronavirus cases have fallen, allowing consumers and businesses to return to normal.