A former head of US President Donald Trump's election campaign lied to the FBI and special counsel Robert Mueller's office after he reached a plea agreement with prosecutors, the special counsel's office said in a court filing Monday.

Paul Manafort agreed to cooperate with the investigation into Russian interference in the 2016 presidential election in September after he was convicted of separate charges related to his time as political consultant in Ukraine.

On Monday, the special counsel's office accused the one-time campaign chairman of lying to investigators "on a variety of subject matters." 

The prosecutors did not provide further details on the substance of the accusations, saying they would detail the accusations in a later court filing.

In the same filing, Manafort's defence attorneys said the 69-year-old "believes he has provided truthful information and does not agree with the government’s characterization or that he has breached the agreement."

The prosecutors said they are no longer beholden to the plea deal in which they were expected to recommend leniency for Manafort's sentencing.

Both the prosecution and the defence asked the court not to delay sentencing.

The Republican political operative, who headed Trump's campaign for two months in mid-2016, pleaded guilty to conspiring to defraud the US and conspiring to obstruct justice in connection with his work as political operative in Ukraine.

Also on Monday, ex-Trump campaign aid George Papadopoulos began his 14-day prison sentence for lying to FBI investigators in connection with the Russia investigation.