Donald Trump's former lawyer Michael Cohen has pleaded guilty to lying to the investigation into whether the Trump campaign colluded with Russia during the 2016 presidential election.

Cohen made a surprise appearance in a New York courtroom at around 9am on Thursday Eastern Time (1am AEDT) to enter the plea.

He admitted to making false statements in 2017 to the US Senate Select Committee on Intelligence about a plan to build a Trump Tower in Moscow.

Mr Trump lashed Cohen outside the White House, telling reporters his former employee was a "weak person" who was "lying to get a reduced sentence".

He said he had decided not to pursue the business deal "everybody knew about" but "there would be nothing wrong if I did do it."

Mr Trump said his former lawyer was not a threat to his presidency. "He was convicted of various things unrelated to us," said the President.

Cohen previously said the deal was stopped in January 2016, but the court was told discussions with Russia continued until at least June that year, when Mr Trump was the presumptive Republican presidential nominee.

The lawyer said he discussed the deal with Mr Trump more than three times and briefed his family members on the project, according to court documents. Cohen said he took steps in contemplation of Mr Trump travelling to Russia, and discussed it with his employer.

Cohen and his lawyers said he lied "to be loyal" to Mr Trump.

One of the prosecutors working with Special Counsel Robert Mueller was present in the courtroom.

In August, Cohen pleaded guilty to other federal charges involving his taxi businesses, bank fraud and his campaign work for Mr Trump.

This included payments to porn star Stormy Daniels and former Playboy model Karen McDougal, who claim they had affairs with Mr Trump.

The lawyer once said he would "take a bullet" for Mr Trump but has distanced himself from his former employer in recent weeks, as the President repeatedly lashed out at the "witch hunt" investigation.

Cohen has spent more than 70 hours with Mr Mueller's investigative team, according to US ABC News. His lawyer, Guy Petrillo, said he would give the court a letter outlining how his client has cooperated with Mr Mueller's investigation.

Cohen gave a statement to congressional committees last year saying the President's company had pursued a project in Moscow during the Republican primary but the plan was abandoned "for a variety of business reasons". Cohen also said he sent an email to the spokesman for Russian President Vladimir Putin as part of the potential deal.

In his statement, he said that he worked on the real estate proposal with Felix Sater, a Russia-born associate who he said claimed to have deep connections in Moscow.

The discussions about the potential development began after Trump had declared his candidacy. Cohen had said the talks ended when he determined that the project was not feasible.

House Speaker Paul Ryan said Cohen "should be prosecuted to the extent of the law. That's why we put people under oath."

It comes as an end-of-year explosion looms to Mr Trump, with the Russia probe reaching its climax and more charges expected. Mr Mueller has already charged 32 people in connection with the probe.

The President finally delivered his written answers to Mr Mueller last week, asserting that he did not know about WikiLeaks or a 2016 Trump Tower meeting between his son, campaign officials and a Russian lawyer promising dirt on Mrs Clinton, according to two sources familiar with the matter.

On Thursday night, it emerged secret charges may be pending against WikiLeaks founder Julian Assange, who is living in self-imposed exile at the Ecuadorean Embassy in London after the country granted him asylum.

The Ecuadorean government removed its pro-Assange UK ambassador last week.

WikiLeaks released the emails of Hillary Clinton campaign chair John Podesta, which authorities allege were stolen by Russia, in October of that year.

Mr Mueller alleged in a Monday night court filing that former Trump adviser Paul Manafort had repeatedly lied to investigators about his links with WikiLeaks.

Mr Manafort met with Mr Assange a few months before WikiLeaks released the hacked emails, according to The Guardian . Mr Manafort and Mr Assange both deny any meetings.

Conservative author Jerome Corsi said Mr Mueller's team was preparing to indict him for lying about an email he sent in August 2016 alerting Trump campaign adviser Roger Stone to upcoming document releases by WikiLeaks.

Mr Corsi allegedly spoke with a London contact at Mr Stone's request, before reporting back that WikiLeaks planned to release damaging material on Mrs Clinton. Mr Corsi said he had rejected a plea deal from the Mueller investigation, insisting he was forgetful rather than deliberately misleading in answering questions about the relationship between WikiLeaks and Mr Stone. He could now face charges.

- With wires