The US special counsel Robert Mueller has accused former Trump campaign chair Paul Manafort of breaching a plea agreement by lying to the FBI.

Mueller is leading the probe into alleged Russian interference in the 2016 US election.

In one of the most significant developments in the investigation to date, Paul Manafort agreed to plead guilty to two crimes in September.

The plea agreement - which meant that Mr Manafort avoided a second trial and had five other charges dropped - stated that Mr Manafort would "cooperate fully, truthfully, completely and forthrightly [...] in any and all matters as to which the government deems the cooperation relevant".

However, Mr Manafort is now alleged to have lied after the agreement - although his legal team disputes the accusation.

In a court filing, the special counsel's office said: "After signing the plea agreement, Manafort committed federal crimes by lying to the Federal Bureau of Investigation and the Special Counsel’s Office on a variety of subject matters, which constitute breaches of the agreement.

"The government will file a detailed sentencing submission [...] in advance of sentencing that sets forth the nature of the defendant’s crimes and lies, including those after signing the plea agreement."

However, Mr Manafort's legal team insisted their client has met with the government on several occasions and answered their questions.

They state: "Manafort has provided information to the government in an effort to live up to his cooperation obligations.

"He believes he has provided truthful information and does not agree with the government’s characterisation or that he has breached the agreement."

Both parties have called for a swift sentencing date.

The latest developments in the probe appeared to provoke further criticism of the investigation from Donald Trump himself, who has repeatedly claimed the investigation is a "witch hunt".

Taking to Twitter, President Trump claimed: "Mueller and his gang of Angry Dems are only looking at one side, not the other.

"Wait until it comes out how horribly & viciously they are treating people, ruining lives for them refusing to lie. Mueller is a conflicted prosecutor gone rogue."

....The Fake News Media builds Bob Mueller up as a Saint, when in actuality he is the exact opposite. He is doing TREMENDOUS damage to our Criminal Justice System, where he is only looking at one side and not the other. Heroes will come of this, and it won’t be Mueller and his...

— Donald J. Trump (@realDonaldTrump) November 27, 2018