Democratic presidential contender Joe Biden has called for the impeachment of US President Donald Trump for the first time, in his strongest indictment of the Republican leader's actions involving Ukraine.

Key points: Mr Biden said Mr Trump convicted himself by refusing to reply to a congressional inquiry

Mr Biden said Mr Trump convicted himself by refusing to reply to a congressional inquiry Mr Trump said in a tweet he did nothing wrong

Mr Trump said in a tweet he did nothing wrong The White House declared on Tuesday its refusal to cooperate with the probe

Mr Biden, who featured prominently in the actions that led congressional Democrats to start impeachment proceedings, had earlier refrained from making an outright plea for the move.

He said in an opinion piece over the weekend they would let Congress do its job on impeachment.

During a campaign stop in New Hampshire, Mr Biden — the Democratic frontrunner for the party's nomination to run against Mr Trump in the November 2020 election — took the gloves off.

"With his words and his actions, President Trump has indicted himself. By obstructing justice, refusing to reply with a congressional inquiry, he's already convicted himself," he said.

"In full view of the world and the American people, Donald Trump has violated his oath of office, betrayed this nation and committed impeachable acts.

"To preserve our constitution, our democracy, our basic integrity, he should be impeached."

Mr Trump responded in real time on Twitter, using one of his nicknames for Mr Biden.

"So pathetic to see Sleepy Joe Biden, who with his son, Hunter, and to the detriment of the American taxpayer, has ripped off at least two countries for millions of dollars, calling for my impeachment — and I did nothing wrong."

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The Democratic-led House of Representatives opened impeachment proceedings against Mr Trump on September 24, over a July phone call he had with Ukrainian President Volodymyr Zelenskiy in which he asked him to investigate Mr Biden and his businessman son, Hunter Biden.

Despite Mr Trump's allegations that Mr Biden engaged in improper dealings in Ukraine, made without providing evidence, there are few signs the controversy has damaged the Democratic former vice president's 2020 prospects.

Public opinion polls have shown Mr Biden's support remaining relatively stable.

President Donald Trump said in a tweet he didn't do anything wrong. ( AP: Manuel Balce Ceneta )

US Secretary of State Mike Pompeo said he listened to the July 25 call between Mr Trump and Mr Zelenskiy and it was "wholly appropriate" for the President to ask Ukraine's leader to investigate corruption in his country.

"I found that to be wholly appropriate to try to get another country to stop being corrupt," Mr Pompeo said in an interview on PBS's NewsHour program.

The White House on Tuesday declared its refusal to cooperate with the impeachment probe, describing it as a partisan attempt to hobble the President.

The three congressional committees leading the inquiry were working on final arrangements on Wednesday to interview a US intelligence officer who filed a whistle blower complaint that led to their investigation.

Vice-President Mike Pence said he was working with the White House counsel's office to release transcripts of his own calls with Mr Zelenskiy.

Mr Pence said records of his two phone conversations would help exonerate Mr Trump of any wrongdoing.

Mr Pence was asked about releasing his transcripts and told reporters, "I'd have no objection to that."

He spoke after an event in Waukee, Iowa, where he addressed supporters about the President's trade policy.

Mr Pence said he "never discussed the issue of the Bidens" with Mr Zelenskiy.

Mike Pence says records of his two phone conversations with Mr Zelenskiy will help exonerate Mr Trump. ( Reuters: Joshua Roberts )

He again defended the President, insisting that a "plain reading" of the rough transcript of Mr Trump's call with the Ukranian leader showed "there was no quid pro quo".

The State Department this week abruptly blocked the US ambassador to the European Union, who had been in touch with Ukrainian officials on Mr Trump's behalf, from speaking to the inquiry.

The investigation is focused on whether Mr Trump used almost $US400 million ($594 million) in congressionally approved aid to Ukraine as leverage to pressure Mr Zelenskiy to begin an investigation of the Bidens.

Mr Trump has denied wrongdoing and defended a July 25 phone call to Mr Zelenskiy.

Reuters/AP