The most startling part of Washington's sanctions on Russian businessmen loyal to President Vladimir Putin may be a single sentence that contains an explosive allegation: that Putin himself profits from the world's No 4 oil trading company, Gunvor.

Among the people the US sanctioned on Thursday as part of its drive to put pressure on Russia for its intervention in Ukraine was businessman Gennady Timchenko, a long-time acquaintance of Putin and, until this week, co-owner of Geneva-based Gunvor, which trades nearly 3 percent of the world's oil.

In announcing the sanctions, the Treasury Department went a step further, adding a single sentence that hits squarely at one of the most controversial topics that Putin has faced in 13 years as the Kremlin ruler and head of the government.

"Timchenko activities in the energy sector have been directly linked to Putin. Putin has investments in Gunvor and may have access to Gunvor funds," the statement said.

The US Treasury Department declined further comment on what information it has about Putin's investments in Gunvor and how exactly he could have access to Gunvor's funds.

It provoked a quick and furious response. Gunvor said the statement was "outrageous" and "blatantly false."

"Gunvor categorically denies that Vladimir Putin has or has ever had any ownership or that he is a beneficiary of our business directly or indirectly," Gunvor said.

Gunvor has repeatedly said that Timchenko and chief executive officer Torbjorn Tornqvist had equal stakes each of around 45 percent and the remaining 10 percent was owned by the staff.

Timchenko has repeatedly denied that Putin helped him create his vast business empire, yet because of his long and close relationship with Putin speculation has persisted.

Putin's spokesman Dmitry Peskov called the sanctions "unacceptable" and said the Kremlin was studying the impact of the inclusion of Timchenko on the list.

He did not comment on the allegations that Putin has investments in Gunvor.

Later in the day a Treasury official clarified that Timchenko's designation would not affect Gunvor since Timchenko owned less than 50 percent.

At the same time, Gunvor announced that Timchenko had sold his share to Tornqvist on Wednesday, a day earlier. Tornqvist now owns 87 percent, while 13 percent belongs to the employees.

Still, the growing concern among analysts is that the accusation, and the direct blow at Putin's closest allies, may provoke an even more dramatic response from Moscow - potentially even using its vast energy supplies as a weapon against the West.

Putin said this month he believed the West "crossed the line" in Ukraine after leaked audio recordings have shown US diplomats discussing how to oust Kremlin-backed Ukrainian President Viktor Yanukovich and which pro-Western politicians should form the new government.

Russia responded on Thursday by sanctioning several US politicians but analysts said more will likely follow after US accusations about Putin's personal enrichment.

"We expect asymmetrical moves by Moscow in coming days," said think-tank and risk consultants Eurasia group. "These could be in the security area (suspending an arms control agreement) or they could involve measures against US business."

Russia supplies a third of Europe's gas and between a fifth and a fourth of its oil. It has previously twice suspended gas supplies to Ukraine and Europe following pricing spats with Kiev.

DENIALS

The wording of the statement goes far beyond anything that has been said previously about Putin. He has faced accusations for years, typically from political opponents, that he helped Timchenko create the Gunvor empire.

None have ever provided any evidence, and no media outlets have ever reported any proof of ownership interests in Gunvor.

Putin worked in the mayor's office in the early 1990s when Timchenko and his friends, Putin said, spun off an oil trading unit of the Kirishi oil refinery.

Gunvor grew spectacularly since then with a turnover of $93 billion in 2012 compared with $5 billion in 2004. It traded large volumes of oil of Russian state companies such as Rosneft at the end of last decade but since then ceded its leading positions and now focuses on trading in Europe and Asia.

According to WikiLeaks cables, released in 2010, John Beyrle, the former US ambassador to Moscow, reported that Gunvor's "secretive ownership is rumoured to include prime minister Putin."

A call made by Reuters to Eurasia Foundation, where Beyrle works now after leaving the foreign service, was not answered late on Thursday evening.

The first time Putin addressed the issue directly was in 2011.

"I assure you, I know that a lot is being written about it, without any participation on my part... I have known the citizen Timchenko for a very long time, since my work in St Petersburg," Putin told a group of Russian writers.

"I never interfered with anything related to his business interests. I hope he will not stick his nose into my business either," Putin said.

Putin has repeatedly said that he has read in the press reports about his immense wealth and that he was even the world's richest man but denied those reports as nonsense.

"Yes it is true. I'm not only the richest man in Europe but in the whole world. I'm collecting emotions," he told reporters in 2008. Talk of vast personal riches "is all rubbish."

KILLING THE RUMOR?

In response to the statement from Washington, Gunvor said its ownership structure was confirmed during its 2013 $500 million bond issue, co-led by Goldman Sachs.

"They can attest to the fact that, although not required, Gunvor undertook an enhanced level of due diligence in our prospectus, normally only required in the US to demonstrate confidence in our business and in acknowledgement of the high level of scrutiny we withstand," a Gunvor's spokesman said.

Goldman said it could not immediately comment on the issue. - Reuters