The White House press secretary Sarah Sanders has been interviewed by US Special Counsel, Robert Mueller.

She told CNN: "The president urged me, like he has everyone in the administration, to fully cooperate with the special counsel.

"I was happy to voluntarily sit down with them", she added.

Mr Mueller's investigation into Russian interference in the 2016 US presidential election and possible collusion between Mr Trump's campaign and Moscow is ongoing.

President Trump repeatedly has denied collusion with Russia and criticised the Mueller investigation as a "witch hunt."

The interview is thought to have happened around the same time as the special counsel interviewed former White House chief of staff John Kelly.

Mueller was also involved in the case against the former lawyer of US President Donald Trump, Michael Cohen.

He has been sentenced for crimes including tax evasion, lying to Congress and arranging illicit payments to silence women who threatened Mr Trump's presidential campaign in 2016.

The lying to Congress charge was brought by Mueller.

A sentencing memo concerning Cohen, released by Mueller, said the lawyer committed a 'serious crime' by withholding information and lying to Congress - including about a proposed Trump-branded hotel project in Moscow.

However, it added that Cohen had taken 'significant steps to mitigate his criminal conduct'.

According to Mr Mueller's document, the lawyer provided information about his own contacts with Russian interests, as well as "information about attempts by other Russian nationals to reach the campaign".