Image copyright Getty Images Image caption Mike Pompeo is yet to publicly respond to the House Democrats' demand

US Secretary of State Mike Pompeo has been ordered by Democrats to turn over documents relating to the Trump administration's dealings with Ukraine.

In a letter, the heads of three House committees subpoenaed Mr Pompeo to produce the documents within a week.

It is the latest move in rapidly escalating impeachment proceedings against President Donald Trump.

He is being scrutinised for pushing Ukraine's president to investigate his Democratic rival Joe Biden.

In a separate development on Friday, the US special envoy for Ukraine negotiations, Kurt Volker, resigned, according to US media reports.

Washington has been rocked by a complaint from an intelligence whistleblower that Mr Trump was "using the power of his office to solicit interference from a foreign country in the 2020 US election".

Mr Trump had denied putting any pressure on Ukrainian President Volodymyr Zelensky in a phone call in July, when Mr Biden was leading polls to win the Democratic nomination for the White House race.

But a transcript of the call released by the White House showed that the president did press Mr Zelensky to investigate Mr Biden.

Mr Trump has alleged that Mr Biden pressed for the sacking of Ukrainian prosecutor Viktor Shokin in 2016 to protect a business that employed his son, Hunter Biden.

Mr Biden did call for the sacking of Mr Shokin, even threatening to withhold $1bn (£813m) in aid to Ukraine. But so did a number of other Western officials who saw Mr Shokin as a hindrance to anti-corruption investigations.

Impeachment is a rarely exercised two-stage political process by which a US president can be removed from office for wrongdoing.

Even if President Trump was impeached by the House of Representatives, he is unlikely to be forced out of the White House because Republicans control the Senate.

The US president has dismissed the impeachment proceedings as a "hoax" and "another witch-hunt".

What is the Pompeo subpoena about?

The subpoena was issued in a joint letter by the House's Foreign Affairs, Intelligence and Oversight committees.

The committees are headed by Elliot Engel, Adam Schiff and Elijah Cummings, respectively.

Media playback is unsupported on your device Media caption Did Trump ask for "a favour" in return for aid to Ukraine?

In the letter, the committees said they were investigating "the extent to which President Trump jeopardized national security by pressing Ukraine to interfere with our 2020 election and by withholding security assistance provided by Congress to help Ukraine counter Russian aggression".

They added that the subpoena was issued because Mr Pompeo had failed to comply with demands to provide relevant documents.

They said his refusal "impairs Congress' ability to fulfil its constitutional responsibilities to protect our national security and the integrity of our democracy".

The three committees also scheduled testimony from five other State Department officials - including Mr Volker and the former US ambassador to Ukraine, Marie Yovanovitch.

Mr Pompeo is yet to publicly comment on the subpoena.

How did we get here?

Democrats began a formal impeachment inquiry earlier this week amid allegations that the president had abused the power of his office to help with his re-election.

The whistleblower's complaint, which was released on Thursday, characterises the president's conduct as a "serious or flagrant problem, abuse, or violation of law".

Media playback is unsupported on your device Media caption US citizens react to the latest developments

The whistleblower said they had learned from several sources that senior White House officials had intervened to "lock down" all details of the call with the Ukrainian president, particularly an official word-for-word transcript.

"This set of actions underscored to me that White House officials understood the gravity of what had transpired in the call," the whistleblower wrote.

The whistleblower's identity remains unknown, but several media outlets have identified them as a CIA officer.

What is the claim about Joe Biden?

On 25 July, Mr Trump raised Mr Shokin's removal during a phone call with Ukraine's newly-elected president - details of which were released by the White House following the whistleblower's complaint.

Mr Trump went on to discuss Hunter Biden and the unsubstantiated allegation that his father - then the US vice-president - stopped an investigation into his son's employer by lobbying Ukraine to fire Mr Shokin.

Media playback is unsupported on your device Media caption What we know about Biden-Ukraine corruption claims

The chief prosecutor's office had an open inquiry into Burisma, a natural gas company on which Hunter Biden was a board member.

There is no evidence of any wrongdoing by the Bidens.

In a BBC interview, former Ukrainian Foreign Minister Pavlo Klimkin said Mr Shokin was sacked for corruption, denying Mr Trump's claims.

Who is Kurt Volker?

Appointed in 2017 in a volunteer capacity, Mr Volker was a key player in US efforts to help resolve an ongoing crisis in Ukraine that started with the annexation of Crimea by Russia and Moscow's support for separatists in the east.

Image copyright EPA Image caption Kurt Volker is also executive director of the McCain Institute for International Leadership think tank

Mr Volker was mentioned in the whistleblower's complaint on 12 August.

It says that Mr Volker and the US ambassador to the EU, Gordon Sondland, met President Zelensky and other Ukrainian politicians on 26 July.

The complaint says Mr Volker and Mr Sondland "reportedly provided advice to the Ukrainian leadership about how to 'navigate' the demands that [President Trump] had made of Mr Zelensky".

Mr Volker and Mr Sondland are also said to have spoken with Rudy Giuliani, Mr Trump's lawyer, to try to "contain the damage" to US national security.

Why is the July phone call controversial?

Democrats accuse Mr Trump of illegally seeking foreign help in the hope of smearing Mr Biden.

Mr Trump has acknowledged that he personally blocked nearly $400m (£324m) in military aid to Ukraine days before the call, but denied that it was to pressure the Ukrainian leader into investigating Mr Biden.

The aid package has since been released.

According to the transcript, the US president called on the Ukrainian leader to talk to US Attorney General William Barr and Mr Trump's lawyer Rudy Giuliani about investigating Hunter Biden's past business dealings.