WASHINGTON (AFP) - United States authorities are investigating a member of Russian President Vladimir Putin's inner circle for money-laundering, The Wall Street Journal reported on Wednesday.

In a politically thorny tightening of screws on Mr Putin's billionaire allies, the Justice Department and New York state prosecutors are looking into whether Mr Gennady Timchenko transferred funds linked to allegedly corrupt deals in Russia through the US financial system, the Journal reported, citing a number of unnamed people familiar with the case.

"The prosecutors are probing transactions in which the Geneva-based commodities firm Mr Timchenko founded, Gunvor Group, purchased oil from Russia's OAO Rosneft and later sold it to third parties," the report said. "Investigators have in recent months requested information about the prices Gunvor charged," it added.

While the transactions took place before the US imposed sanctions over Russia's action in Ukraine, the fund transfers could be money laundering if they originated in irregular sales of state assets including oil, the report said.

Mr Timchenko was among the first Russian business heavyweights the United States hit with sanctions after Russia intervened in Ukraine's Crimea region.

Over 4,000 people - most of them civilians - have been killed in the conflict in east Ukraine since April, the United Nations says.