Democrats have threatened to subpoena Robert Mueller and force him to testify, amid an intensifying stand off with the White House.

In what represented the latest piece of sabre-rattling from Democrats seeking to scrutinise an administration trying to put up the shutters, congressman Jerry Nadler told reporters he was confident the special counsel would appear on Capitol Hill, though it will not take place next week, as had originally been anticipated.

“We’re talking to the department of justice. We’re talking to Mueller. Hopefully he will come in. It won’t be next week,” said Mr Nadler, chair of the House of Representative’s judiciary committee.

Mr Nadler said he and his colleagues were hoping not to have to issue a witness summons to force the former FBI Director come to testify about his two-year investigation into Russia’s alleged interference in the 2016 election and possible collusion with the Trump campaign, but that they were prepared to do so if required.

He added: “He will come at some point. If necessary, we will subpoena him and he will come.”

Mueller investigation: The key figures Show all 12 1 /12 Mueller investigation: The key figures Mueller investigation: The key figures Robert Mueller is the special counsel overseeing the investigation into Russia's meddling in the 2016 election, and potential obstruction of justice by the president. Mr Mueller has a pristine reputation in Washington, where he was previously in charge of the FBI. Throughout his investigation, he and his team have been notoriously tight lipped about what they know and where their investigation has led. REUTERS Mueller investigation: The key figures Former FBI director James Comey was the catalyst that led to the appointment of special counsel Robert Mueller. Mr Comey was fired by the president after Mr Trump reportedly asked him to drop his own Russia investigation. Mr Trump has long maintained that the investigation is a "witch hunt". AFP/Getty Images Mueller investigation: The key figures Deputy attorney general Rod Rosenstein had authority over the special counsel investigation for much of the two years it has been active. Mr Rosenstein found himself with that responsibility after then-attorney general Jeff Sessions recused himself from that oversight. AP Mueller investigation: The key figures Attorney general Jeff Sessions's decision to recuse himself from oversight of the special counsel investigation may have cost him his job in the end. Mr Sessions resigned last year, after weathering a contentious relationship with Donald Trump who vocally criticised his attorney general for taking a step back. Mr Sessions recused himself from the oversight citing longstanding Justice Department rules to not be involved in investigations overseeing campaigns that officials were apart of. AP Mueller investigation: The key figures Attorney General William Barr is currently responsible for oversight of the special counsel investigation. Mr Barr's office will be the first to receive the Mueller report when it is finished. His office will then determine what portion or version of that report should be delivered to Congress, and also made public. EPA Mueller investigation: The key figures Michal Cohn is the president's former personal lawyer, who has been helping the special counsel investigation as a part of a plea deal over financial crimes, and campaign finance crimes, he has pleaded guilty to. Among those crimes, Cohen admitted to facilitating $130,000 in hush money payments to adult film actress Stormy Daniels during the 2016 campaign. Cohen has said he did so at the direction of Mr Trump. Cohen has also admitted that he maintained contacts with Russian officials about a potential Trump real estate project in Moscow for months longer than Mr Trump and others admitted. The talks continued well into 2016 during the campaign, he has said. AP Mueller investigation: The key figures Stormy Daniels has alleged that she had an affair with Donald Trump in 2006, soon after Melania Trump gave birth to Baron Trump. The accusation is of particular importance as a result of the $130,000 hush money payment she received to keep quiet about the affair during the 2016 campaign. AP Mueller investigation: The key figures Paul Manafort was Donald Trump's former campaign chairman. Manafort was charged alongside Rick Gates for a slew of financial crimes, and was convicted on several counts in a Virginia court. He then pleaded guilty to separate charges filed in a Washington court. Manafort has been sentenced to just 7.5 years in prison for his crimes — in spite of recommendations from the special counsel's office for a much harsher sentence. AP Mueller investigation: The key figures George Papadopoulos was one of the first individuals associated with the Trump campaign to be charged by the Mueller probe. He ultimately received a 14 day prison sentence for lying to investigators about contacts he had with Russian officials. AP Mueller investigation: The key figures Roger Stone is a well known political fixer and operative, who has made a name for himself for some dirty tactics. He has been charged by the Mueller probe earlier this year, and he has been said to have had prior knowledge that WikiLeaks planned on publishing stolen emails from the Hillary Clinton campaign in 2016. Getty Images Mueller investigation: The key figures Rick Gates was charged alongside former Trump campaign chairman Paul Manafort for a range of crimes. Gates, who worked alongside Manafort for a pro-Russia Ukrainian political party. The two were charged with conspiracy and financial crimes. Gates pleaded guilty. AP Mueller investigation: The key figures Former national security adviser Michael Flynn was one of the first casualties of the Russia scandal, and was forced out of his position in the White House weeks after Donald Trump took office. Flynn pleaded guilty in 2017 to "willfully" making fraudulent statements about contacts he had with Russian officials including former Russian ambassador to the US Sergey Kislyak. Flynn then lied to Vice President Mike Pence about that contact. REUTERS

Following the release of Mr Mueller’s report, which said it found no collusion between the Trump campaign and Russia but did not exonerate the president on the question of potential obstruction of justice, Democrats have been seeking to obtain documents they say are being denied to them, and testimony from White House officials.

They requested an unredacted copy of Mr Mueller’s report from William Barr, the attorney general who, with his deputy, decided the report did not make a clear case for indicting Mr Trump for obstruction of justice. They were turned down.

Kellyane Conway refuses to say whether Trump tried to fire Robert Mueller

They have also sought the testimony of former White House counsel Don McGahn. Mr Nadler repeated plans to hold Mr McGahn in contempt if he does not appear before the committee to testify on May 21 under an existing subpoena.

“He knows that if he doesn't testify on the 21st without a court order, which he won't get, he'll be subject to a contempt citation,” he said.

The White House earlier this week had directed Mr McGahn not to comply with an earlier summons, or to provide documents.

Last month, Mr Barr cut short his testimony on Capitol Hill after it was revealed Democrats wanted to have him quizzed by lawyers as well as legislators.

Mr Trump previously said he believed Mr Mueller ought not to testify, but this week changed his stance, claiming the decision was up to Mr Barr.