Media playback is unsupported on your device Media caption Trump impeachment: Was there a quid pro quo?

US lawmakers are preparing to question President Trump's top intelligence official over a whistleblower complaint that sparked an impeachment inquiry.

Acting national intelligence director Joseph Maguire initially refused to share the complaint with Congress.

It refers to a controversial phone call between Mr Trump and the Ukrainian president, US media report.

In the call, Mr Trump pushes Volodymyr Zelensky to investigate corruption claims involving the son of Joe Biden.

Mr Biden - the leading Democratic presidential hopeful - says a rough transcript of the conversation released on Wednesday shows the president betrayed his oath of office.

Democrats opened an impeachment inquiry against the Republican president on Tuesday, accusing Mr Trump of seeking foreign help in the hope of smearing Mr Biden and of using military aid to Ukraine as a potential bargaining tool.

President Trump has dismissed the impeachment proceedings as a "hoax" and a "witch-hunt".

The whistleblower's formal complaint has now been released.

A House of Representatives vote to impeach the president could trigger a trial in the Senate on whether to remove Mr Trump from office. But Republicans control the upper house so it is considered unlikely that a two-thirds majority to dismiss the president could be reached.

Congress's investigation focuses partly on whether Mr Trump abused his presidential powers and sought to help his own re-election by seeking the aid of a foreign government to undermine Mr Biden.

Mr Trump denies putting pressure on Mr Zelensky.

What did Trump say about Biden in the call?

According to notes of the 25 July phone call, President Trump asked his Ukrainian counterpart to look into corruption claims involving the son of Joe Biden, Mr Trump's possible rival in next year's presidential election. There is no evidence of any wrongdoing by the Bidens.

Media playback is unsupported on your device Media caption Trump: 'It was going to be the call from hell. It turned out to be a nothing call'

Mr Trump discusses with newly elected Mr Zelensky the 2016 removal of a prosecutor, Viktor Shokin, according to the notes. He then goes on to discuss Mr Biden's son, Hunter Biden, and the unsubstantiated allegation that Mr Biden stopped the prosecution of his son.

On the call, the US president also asks Mr Zelensky to work with US Attorney General William Barr and Mr Trump's personal lawyer, Rudolph Giuliani, to look into the matter, according to the notes.

The Department of Justice said on Wednesday that Mr Trump had not spoken to the attorney general about having Ukraine investigate Mr Biden, and Mr Barr had not communicated with Ukraine.

What has Mr Zelensky said about the call?

Image copyright EPA Image caption Mr Zelensky and Mr Trump met on the sidelines of the United Nations General Assembly in New York on Wednesday

The Ukrainian leader has said he had thought only Mr Trump's side of their phone call would be published.

"I personally think that sometimes such calls between presidents of independent countries should not be published," he told Ukrainian media in New York, Reuters news agency reports.

Mr Zelensky also said Hunter Biden's case was "one of many" he discusses with foreign leaders.

"For me, this is one of numerous cases that I talk about with the leaders of other states, if they ask me," he is quoted as saying by the Interfax-Ukraine news agency.

"Sometimes, these cases are being investigated or have not been investigated for many years - I do not know why," he added.

What is the claim against Joe Biden?

Mr Trump and his conservative allies have focused on how Mr Biden, as US vice-president in 2016, lobbied Ukraine to fire Mr Shokin.

Mr Shokin's office had opened an investigation into Burisma, a natural gas company on which Hunter Biden was a board member.

Other Western officials had called for Mr Shokin to be fired because of the perception that he was soft on corruption.

Mr Biden last year told a foreign policy event how he threatened to withhold $1bn in aid to Ukraine unless Mr Shokin was removed.

How the controversy unfolded