US President Donald Trump has swatted away mounting pressure from Democrats demanding his impeachment, rejecting accusations he had offered aid to Ukraine only if it investigated his political rival Joe Biden.

Battered by the burgeoning scandal during his first full day at the United Nations General Assembly in New York, Mr Trump tried to shift the controversy towards Mr Biden, accusing the former vice president, without evidence, of engaging in corruption in Ukraine.

Democrats have fumed as Mr Trump's administration has blocked Congress from obtaining a whistleblower's secret complaint allegedly detailing the president's actions, and they ramped up their demands for the document that sparked the latest crisis.

The complaint reportedly centres on Mr Trump's July phone call with Ukrainian counterpart Volodymyr Zelensky, and a possible attempt to coerce him into digging up damning information about Mr Biden's son's business dealings in Ukraine.

In the days before the phone call, Mr Trump ordered his chief of staff to withhold almost $400m in military aid earmarked for Ukraine, the Washington Post and New York Times reported, quoting senior administration officials.

The officials were instructed to tell politicians the freeze was due to an "interagency process", but to provide no additional information, the Post reported.

Mr Trump acknowledged addressing alleged corruption involving Mr Biden and son Hunter on the call.

"Joe Biden and his son are corrupt," he said, providing few details other than to say Hunter Biden, who once served on a Ukrainian natural gas company's board, "took money from Ukraine".

Joe Biden and son Hunter, along with granddaughter Finnegan

He also insisted that, in his call with the Ukrainians, "I put no pressure on them whatsoever," and "I did not make a statement that you have to do this or I won't give you aid".

Mr Biden fired back on Twitter: "So release the transcript of the call then."

To date, there has been no evidence of illegal conduct in Ukraine by the Bidens. 

House Speaker Nancy Pelosi, who so far has hesitated to start impeachment, signaled that could change.

If the administration does not produce the whistleblower complaint, "they will be entering a grave new chapter of lawlessness which will take us into a whole new stage of investigation," she said.

The Democratic chairs of three key intelligence-related House committees threatened subpoenas against Secretary of State Mike Pompeo if he does not produce documents related to a meeting between Mr Trump's lawyer Rudy Giuliani and Ukrainian officials.

The three said seeking to enlist a foreign actor to interfere with elections undermines US sovereignty and democracy, "yet the president and his personal attorney now appear to be openly engaging in precisely this type of abuse of power involving the Ukrainian government ahead of the 2020 election".

Ukrainian President Volodymyr Zelensky

Several Democrats argue that Mr Trump's call for Ukraine to investigate Joe Biden, and what they suspect was a threat to condition aid to Ukraine on the country doing so, is impeachable conduct.

That view may be pushing House leaders toward a tipping point for launching removal proceedings, along with their Democratic Senate colleagues.

Mr Trump "has fundamentally corrupted his office," Democratic Senator Chris Murphy told reporters, accusing Republicans of being "complicit in that corruption every single day that they stand with him".

Last night, seven new House Democrats announced their support for impeachment in an essay published in The Washington Post.

"If these allegations are true, we believe these actions represent an impeachable offense," wrote the representatives, all of whom are veterans of the military or intelligence community.

The Senate's Republican leader Mitch McConnell said his chamber's intelligence committee was launching a bipartisan inquiry into the whistleblower complaint.

That move drew praise from some Republicans, while others insisted that Democrats were on a fishing expedition.

"The Democrats are cranking up the outrage machine again, beating the impeachment drum," Senator John Barrasso said. 

"They're hoping they have something here. I just don't see it."

With pressure building, a handful of Republicans in the Senate, which would put Mr Trump on trial should the House impeach him, have indicated they want the president to be more transparent.

But most congressional Republicans have either defended the president or remained silent.

Senator Marco Rubio acknowledged it was inappropriate for Mr Trump to discuss his rival on a call with Ukraine's president, but said that was "different" from being an impeachable offence.

All eyes will be on Washington on Thursday, when the administration official who blocked congressional review of the whistleblower document, acting Director of National Intelligence Joseph Maguire, testifies publicly before a House of Representatives panel.

Launching impeachment proceedings could be politically risky ahead of an election, especially given the high hurdle of convicting the president in the Republican-led Senate.

Of the 235 House Democrats, 154 of them, plus Republican-turned-independent Justin Amash, support launching an impeachment inquiry, according to Politico.