An official of the United States government has, for what is believed to be the first time, directly accused Russian President Vladimir Putin of corruption, in a BBC programme that aired Monday.

The US government imposed sanctions against a number of Kremlin insiders in 2014 after Russia's annexation of Crimea in Ukraine, but did not accuse Putin of direct involvement in corruption.

However, during a Panorama investigation into Putin's "secret riches", the acting under secretary for terrorism and financial intelligence at the US Treasury said Putin was a "picture of corruption."

"We've seen him enriching his friends, his close allies, and marginalising those who he doesn't view as friends using state assets," Adam Szubin, who oversees US Treasury sanctions, told the Panorama programme.

"Whether that's Russia's energy wealth, whether it's other state contracts, he directs those to whom he believes will serve him and excludes those who don't. To me, that is a picture of corruption."

The US government has know about this for "many, many years", he added.

The programme cited a secret CIA report from 2007 stating that Putin's wealth stood at around $40 billion (37 billion euros).

"He supposedly draws a state salary of something like $110,000 a year," said Szubin. "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."

Putin's spokesman told the BBC that "none of these questions or issues needs to be answered, as they are pure fiction".

Moscow also on Thursday dismissed as a "joke" a British inquiry's findings that Putin "probably approved" the killing of ex-spy Alexander Litvinenko a decade ago in London.