Media playback is unsupported on your device Media caption What does it take to impeach a president?

There are growing Democrat calls for impeachment proceedings against President Donald Trump, after a former White House counsel defied a legal order to appear before Congress.

Donald McGahn skipped a hearing on Tuesday about the Mueller report.

Mr Trump has vowed to block all subpoenas of current and former staff, sparking outrage from the Democrats.

House Speaker Nancy Pelosi has scheduled a party meeting on Wednesday to discuss impeachment.

The Trump administration says the report by Special Counsel Robert Mueller into allegations of Russian collusion and obstruction of justice cleared the president of wrongdoing, and that there are no further questions to answer.

In a tweet, President Trump argued he had "allowed everyone to testify" to Robert Mueller's team and accused the Democrats of seeking a "do-over" of the investigation.

On the subpoenas - a witness summons - the White House and the Department of Justice both say staff cannot be forced to testify.

But House Judiciary Committee chairman Jerry Nadler said their subpoenas "are not optional" and said they would hear Mr McGahn's testimony "even if we have to go to court to secure it".

Image copyright AFP Image caption Don McGahn left the White House in October

Democratic Party leaders have so far held off pressure from their lawmakers to begin impeachment proceedings, and the party is divided over the merits of the move, but the pressure is growing as the president stonewalls congressional inquiries.

Democratic representative Mark Pocan - one of the leaders of the party's progressive wing - said the president's latest moves had enhanced his "appearance of guilt" and "pushed Congress to a point where we must start an impeachment inquiry".

"We need to do our job and vote on impeachment," tweeted Representative Alexandria Ocasio-Cortez.

And Republican congressman Justin Amash broke ranks, tweeting that the president's behaviour met "the threshold for impeachment" but he has been criticised by fellow party members.

Asked about impeachment on Tuesday, Democratic House Majority Leader Steny Hoyer, said: "I don't think we're there at this point in time."

Why did McGahn not testify?

A letter to the House of Representatives Judiciary Committee said Mr McGahn was "absolutely immune from compelled Congressional testimony" and said Mr Trump had directed him not to appear on Tuesday.

Mr McGahn - who resigned after almost two years as White House counsel in October 2018 - said he would respect the president's instruction.

In the wake of Mr McGahn's failure to appear, Mr Nadler announced he had issued subpoenas to former White House Communications Director Hope Hicks and Annie Donaldson, Mr McGahn's former chief of staff.

According to a New York Times report, Democratic lawmakers and aides were examining possible rules changes to allow the House to fine people held in contempt, as well as other means to break the impasse.

Presidential administrations have challenged subpoenas before, but analysts say President Trump's vow to block all subpoenas issued by the House Judiciary Committee in relation to the Mueller report is unprecedented.

What is this about?

The subpoena for Mr McGahn's testimony is part of a wider inquiry by Congressional Democrats into Mr Trump's alleged obstruction and abuse of power.

In March, the House Judiciary Committee issued document requests related to the investigation of 81 people and groups.

Mr McGahn was interviewed for 30 hours by Mr Mueller's team of investigators, and was frequently cited in their 448-page report, released in April.

Media playback is unsupported on your device Media caption The Mueller report - in 60 seconds

Mr Trump later said he had authorised this co-operation with Mr Mueller.

The Mueller report detailed how Mr McGahn felt the president had pressured him to fire Mr Mueller and, later, write a memo saying that Mr Trump had issued no such directive.

US Attorney General William Barr was questioned this month about the matter by Senate Democrats.

Mr Barr said the president had only suggested that Mr Mueller be "replaced" because of a perceived conflict of interest - and then instructed Mr McGahn to correct inaccurate media reports.

Mr McGahn left the White House in October to return to a Washington law firm, Jones Day, which represents the Trump campaign.