Democrats have ordered US Secretary of State Mike Pompeo to hand over Ukraine-related documents and scheduling testimony for witnesses to alleged abuse of power by US President Donald Trump.

The heads of three committees of the US House of Representatives have given Mr Pompeo one week to produce the documents.

They say multiple State Department officials have direct knowledge of Mr Trump's alleged efforts to enlist the Ukrainian government's help in his bid for re-election next year.

They also announced interviews scheduled with five State officials, including former ambassador to Ukraine, Masha Yovanovitch.

Mr Trump reportedly forced out Ms Yovanovitch earlier this year for resisting his efforts to pressure Ukraine to investigate the Democratic party hopeful for president Joe Biden.

They said: "The committees are investigating the extent to which President Trump jeopardised 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."

House Speaker Nancy Pelosi declared that the impeachment investigation would move quickly, saying the evidence from an intelligence whistleblower's complaint against Mr Trump of abuse of power and an attempted cover-up was unambiguous.

She said: "The clarity of the president's actions is compelling and gave us no choice but to move forward.

"This is about the national security of our country: The President of the United States being disloyal to his oath of office, jeopardising our national security, and jeopardising the integrity of our elections."

President Trump has strenuously denied wrongdoing, calling the claims a "hoax" and the impeachment inquiry "another witch-hunt". 

In a series of Tweets, he criticised Democrats, including Adam Schiff, the representative named by Ms Pelosi to lead the impeachment inquiry, calling them liars.

In a video leaked from a private gathering the president held on Thursday with US diplomats in New York, he made clear he was battling for his survival.

In the video, obtained by Bloomberg, Mr Trump says: "We're at war. These people are sick."

Read: Transcript confirms Trump asked Ukraine leader to investigate Biden

The impeachment proceedings began after the release of the anonymous whistleblower's complaint, reportedly made by a CIA analyst who had worked at the White House.

It accused Mr Trump of pressuring Ukraine's president, Volodymyr Zelensky, in a phone call on 25 July to supply information on Mr Biden, who is  currently the favourite to secure the Democratic nomination and challenge for the presidency next year.

The complaint also revealed that White House aides, alarmed at Mr Trump's implicit offer to release aid in exchange for Mr Zelensky's help, sought to hide the record of the call in a highly secure computer system normally used only for the country's most top-secret intelligence.

More than 300 high-level professionals from the national security community have signed a letter supporting the impeachment investigation.

In a separate development, the US special representative on Ukraine, Kurt Volker, has resigned after Congress ordered him to answer questions in the impeachment investigation.

The whistleblower's complaint said Mr Volker had met senior Ukrainian officials about how to "navigate" President Trump's demands of Mr Zelensky.

House committees have ordered Mr Volker to appear next Thursday to answer questions.

They pointed to a tweet by Rudy Giuliani, President Trump's personal lawyer, in which he showed a screenshot of a conversation in which Mr Volker spoke of connecting him with a top adviser to President Zelensky.