The Kremlin has called on the US Treasury to come up with proof after it told a BBC investigation it considered President Vladimir Putin to be corrupt.
Spokesman Dmitry Peskov told reporters the allegation was an “official accusation” and a “total fabrication”.
Adam Szubin, who oversees US Treasury sanctions, told BBC Panorama that the US government had known Mr Putin was corrupt for “many, many years”.
It is thought to be the first time the US has made such a direct accusation.
Washington has already imposed sanctions on Mr Putin’s aides, but has stopped short of levelling corruption allegations at the president himself.
US restrictions were placed on a number of Kremlin insiders in 2014, after President Putin ordered the annexation of Crimea from Ukraine and conflict broke out in eastern Ukraine. The EU imposed similar measures against Russian companies and individuals, focusing on sectors of the Russian economy that were close to the elite.
The US government stated at the time that President Putin had secret investments in the energy sector.
Mr Peskov told reporters in Moscow that the Panorama allegations would have looked like “another classic case of irresponsible journalism, if not for an official comment from a representative of the US finance ministry”.
As such it was an official accusation. “It clearly shows who is directing this,” said Mr Peskov, who added that such an allegation required proof, to show that the statements were not unfounded slander.
In the programme, Mr Szubin spoke of how “we’ve seen [Mr Putin] enriching his friends, his close allies, and marginalising those who he doesn’t view as friends using state assets”, whether it concerned Russia’s energy wealth or state contracts. “To me, that is a picture of corruption,” he said.
US government officials have been reluctant to be interviewed about President Putin’s wealth, and Mr Szubin would not comment on a secret CIA report from 2007 that estimated it at around $40bn (£28bn).
But he said the Russian president had been amassing secret wealth. “He supposedly draws a state salary of something like $110,000 a year. That is not an accurate statement of the man’s wealth, and he has long time training and practices in terms of how to mask his actual wealth.”
President Putin declined to be interviewed for Panorama but the Kremlin denies such allegations.
In 2008, President Putin personally addressed claims that he was the richest man in Europe, saying: “It’s simply rubbish. They just picked all of it out of someone’s nose and smeared it across their little papers.”
The Panorama programme came days after a UK public inquiry said Mr Putin had “probably” approved the murder of ex-spy Alexander Litvinenko.
Mr Peskov pointed out that the programme had coincided with “quasi-court proceedings” and said that the Kremlin was used to such “false-reporting”, whether it was the result of incompetence or an orchestrated campaign.
Litvinenko, a former Russian Federal Security Service (FSB) agent and fierce critic of Mr Putin, was poisoned in London with radioactive polonium in 2006.
Sir Robert Owen’s report found that Mr Putin was likely to have signed off the attack in part due to personal “antagonism” between the president and Litvinenko. The Russian foreign ministry rejected the report as neither transparent nor unbiased.
Dmitry Peskov, Mr Putin’s spokesman, said Moscow’s official response to the report will happen through “diplomatic channels”, the Russian news agency Interfax was quoted as saying.
Prime Minister David Cameron said the UK would have to go on having “some sort of relationship with them [Russia]” because of the Syria crisis, but it would be done with “clear eyes and a very cold heart”.
The long-awaited report into Mr Litvinenko’s death found that two Russian men – Andrei Lugovoi and Dmitry Kovtun – deliberately poisoned the 43-year-old in London in 2006 by putting the radioactive substance polonium-210 into his drink at a hotel.
Sir Robert Owen, the public inquiry chairman, said he was “sure” Mr Litvinenko’s murder had been carried out by the two men and that they were probably acting under the direction of Moscow’s FSB intelligence service, and approved by the organisation’s chief, Nikolai Patrushev, as well as the Russian president.
He said Mr Litvinenko’s work for British intelligence agencies, his criticism of the FSB and Mr Putin, and his association with other Russian dissidents were possible motives for his killing.
‘Send a message’
There was also “undoubtedly a personal dimension to the antagonism” between Mr Putin and Mr Litvinenko, he said.
The use of polonium-210 was “at the very least a strong indicator of state involvement” as it had to be made in a nuclear reactor, the report said.
The inquiry heard evidence that Mr Litvinenko may have been consigned to a slow death from radiation to “send a message”.
Giving a statement to the House of Commons, Mrs May said Mr Cameron would raise the findings with President Putin at “the next available opportunity”.
She said the UK would impose asset freezes on Mr Lugovoi and Mr Kovtun and that international arrest warrants for the pair remained in place. They both deny killing Mr Litvinenko.
Both men are wanted in the UK for questioning, but Russia has refused to extradite them.
Speaking earlier outside the High Court, Mr Litvinenko’s widow, Marina, said she was “very happy” that “the words my husband spoke on his deathbed when he accused Mr Putin have been proved by an English court”.
She urged the UK government to expel all Russian intelligence operatives, impose economic sanctions on Moscow and impose a travel ban on Mr Putin.
The view from Moscow
By the BBC’s Oleg Boldyrev
For years Moscow rejected allegations of high-level involvement in the murder of Alexander Litvinenko.
The fact President Putin himself is now associated with this assassination has not changed anything.
Taking their lead from Robert Owen’s use of the words “high probability”, the second tier of the Russian establishment, mainly Kremlin-loyalist MPs, are dismissing the entire report as a politically-based fabrication.
Russians on social media are making fun of its conclusions by using the hashtag “PutinProbablyApproved” in Russian – that is #ПутинВозможноОдобрил – to include all manner of crimes.
One Russian MP, Nikolai Kovalev, himself an ex-FSB boss, pointed out relations between Moscow and London would not be harmed by the report as there was no room for making them any worse.
Responding to the report, Mr Lugovoi, who is now a politician in Russia, said the accusations against him were “absurd”, the Russian news agency Interfax was quoted as saying.
“As we expected, there were no surprises,” he said.
“The results of the investigation made public today yet again confirm London’s anti-Russian position, its blinkeredness and the unwillingness of the English to establish the true reason of Litvinenko’s death.”
Mr Kovtun, now a businessman in Russia, said he would not comment on the report until he got more information about its contents, Interfax reported.
London’s Metropolitan Police said the investigation into the “cold and calculated murder” remained ongoing.
Alexander Yakovenko, the Russian ambassador in the UK, said Russia would not accept any decisions reached in secret and based on evidence not tested in open court.
The length of time taken to come to these conclusions led them to believe it was “a whitewash of British security services’ incompetence”, he said.
Mr Yakovenko said these events “can’t help but harm our bilateral relations”.
White House spokesman Josh Earnest said he did not have “any actions” to announce following the inquiry’s findings. “But I certainly wouldn’t rule out future steps,” he said.
By BBC security correspondent, Gordon Corera
The conclusions of this inquiry are stronger than many expected in pointing the finger at Vladimir Putin personally.
The evidence behind that seems to have come from secret intelligence heard in closed session.
Saying that Alexander Litvinenko was killed because he was an enemy of the Russian state will raise pressure on the British government to take real action – the steps taken nearly a decade ago were only limited in scope.
That may pose difficulties given the importance of Russia’s role in the Middle East, but without tough action people may ask if the Russian government has been allowed to get away with what has been described as an act of nuclear terrorism on the streets of London.
Mr Litvinenko fled to the UK in 2000, claiming persecution. He was granted asylum and gained British citizenship several years later.
In the years before his death, he worked as a writer and journalist, becoming a strong critic of the Kremlin.
It is believed he also worked as a consultant for MI6, specialising in Russian organised crime.
The inquiry heard from 62 witnesses in six months of hearings and was shown secret intelligence evidence about Mr Litvinenko and his links with British intelligence agencies.
The Litvinenko case
23 November 2006 – Mr Litvinenko dies three weeks after having tea with former agents Andrei Lugovoi and Dmitri Kovtun in London
22 May 2007 – Britain’s director of public prosecutions decides Mr Lugovoi should be charged with his murder
5 July 2007 – Russia refuses to extradite Mr Lugovoi, saying its constitution does not allow it
May-July 2013 – The inquest into Mr Litvinenko’s death is delayed as the coroner decides a public inquiry would be preferable – but ministers rule out the request
11 February 2014 – High Court rules the Home Office was wrong to rule out an inquiry before the outcome of an inquest