The US Federal Bureau of Investigation has denied a report that it had detained Russian tycoon Roman Abramovich.

"We didn't arrest him. It's just not true," an FBI spokeswoman said in response to an unsourced report by Russia's RBK financial news portal.

The RBK report followed media speculation over the unexplained death in Britain of exiled Russian tycoon Boris Berezovsky.

Mr Berezovsky launched and lost a multi-billion-dollar court case last year against Mr Abramovich, his former business partner.

Mr Abramovich's Moscow spokesman, John Mann, said earlier that the report was false.

"He is in the US but he has not been arrested or detained," Mr Mann said.

The Chelsea FC owner is a major shareholder in London-listed steel firm Evraz, whose shares fell by more than 6% before recovering to trade 3.4% down on the day.

The London-based tycoon is reputed to have close ties to the Kremlin.

Forbes magazine estimates his fortune at $10.2 billion.