U.S. prosecutors have launched a money-laundering investigation of a member of Vladimir Putin's inner circle, several people familiar with the efforts said, in a politically sensitive escalation of pressure on the Russian president’s cadre of billionaire supporters.

The U.S. Attorney’s Office for the Eastern District of New York, aided by the Justice Department, is investigating whether Gennady Timchenko transferred funds linked to allegedly corrupt deals in Russia through the U.S. financial system, the people said.

The prosecutors are probing transactions in which the Geneva-based commodities firm Mr. Timchenko co-founded, Gunvor Group, purchased oil from Russia’s OAO Rosneft and later sold it to third parties, one of the people familiar with the matter said. Investigators have in recent months requested information about the prices Gunvor charged, the person said.

The transactions predate the financial sanctions leveled by the U.S. on Mr. Timchenko and other Putin allies in March in the wake of Russia’s intervention in Ukraine. But transfers of funds related to the transactions could constitute illegal money laundering if the funds were found to have originated from illicit activity such as, for example, irregular sales of state assets like oil.

Mr. Timchenko and Rosneft declined to comment.

Gunvor said it hadn’t been notified of any investigation involving the company itself and couldn’t comment about Mr. Timchenko, who sold his stake in the firm just before the U.S. sanctions were announced. Gunvor, one of the world’s largest trading companies, said it hadn’t purchased crude oil from Rosneft in more than two years. The company denies any wrongdoing regarding the Rosneft tenders.

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