Donald Trump's personal lawyer Rudy Giuliani has been subpoenaed, forcing him to produce documents in a ramping up of the impeachment investigation into the US president.

The Democrat-led inquiry is looking into whether Mr Trump pressured Ukrainian President Volodymyr Zelenskiy during a phone call in July to investigate political rival Joe Biden, days after ordering a freeze of some military help for Kiev.

Mr Giuliani will have to produce the documents by October 15th.

In the subpoena, Representatives Adam Schiff, Elijah E Cummings and Eliot L Engel said Mr Giuliani "admitted on national television that, while serving as the president's personal attorney, he asked the government of Ukraine to target former vice president Joe Biden".

They added that Mr Giuliani also said he is in "possession of evidence - in the form of text messages, phone records, and other communications - indicating that you were not acting alone and that other Trump administration officials may have been involved in this scheme".

Three of Mr Giuliani's business associates - Lev Parnas, Igor Fruman, and Semyon 'Sam' Kislin, who was Mr Giuliani's adviser when he was mayor of New York - have also been asked to provide documents to the investigation.

Mr Trump's impeachment inquiry started after an anonymous whistleblower made a complaint against the president who he claims pressed Mr Zelenskiy to work with Mr Giuliani and Attorney-General William Barr to look into unsubstantiated corruption accusations against former vice president Mr Biden - a leading Democratic contender for the 2020 presidential race.

A White House-released rough transcript of President Donald Trump's July 25th telephone conversation with Ukraine's President Volodymyr Zelenskiy | Image: AP/Press Association Images

The subpoena announcement against Mr Giuliani came less than two hours after Mr Trump defended Mr Zelenskiy and said the whistleblower's report was "incorrect" and his call with the Ukraine president "was perfect".

He accused Mr Schiff of making up what Mr Trump said in the phone call because it was "so good that he couldn't quote from it because there was nothing done wrong".

Following the subpoena, Mr Trump tweeted a video of a pro-Trump reporter saying Mr Schiff misled "millions of people for the sake of making the case for impeachment".

The day before the president demanded to meet the whistleblower.

At the same time as the subpoenas were issued, Republican leader Mitch McConnell said Senate rules mean he would have no choice but to take up any articles of impeachment against Mr Trump if approved by the House.

However, he did not reveal details of what taking up the issue would mean.

On Sunday Adam Schiff, House intelligence committee chairman, said the panel would hear from the secret whistleblower "very soon", but said no date had been set.

Ukraine's president has said his country "cannot be ordered to do anything" when asked about Mr Trump's alleged request to investigate Mr Biden or his son, Hunter.
"We are an independent country," he added.

Mr Zelenskiy criticised the White House's decision to release a memorandum of the July call that sparked the impeachment inquiry, calling it "wrong".