US President Donald Trump on Monday 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, Trump tried to shift the controversy toward Biden, accusing the former vice president, without evidence, of engaging in corruption in Ukraine.

Democrats have fumed as 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.

In a startling admission, Trump acknowledged addressing alleged corruption involving Biden and son Hunter on the call.

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

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."

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 Sunday.

The Democratic chairs of three key intelligence-related House committees on Monday threatened subpoenas against Secretary of State Mike Pompeo if he does not produce documents related to a meeting between 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."

- 'Corrupted his office' -

Several Democrats argue that Trump's call for Ukraine to investigate Biden -- and what they suspect was a threat to condition $250 million in 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.

Democratic Senator Chris Murphy said he has inched closer toward recommending impeachment.

Trump "has fundamentally corrupted his office, and Republicans are complicit in that corruption every single day that they stand with him, every single day that they refuse to get to the bottom of what happened," Murphy told reporters.

As some liberal House Democrats including Rashida Tlaib joined protesters openly calling for Trump's impeachment, 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 Senate Republicans, including Susan Collins.

But others remained insistent 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 Trump on trial should the House impeach him -- have indicated they want the president to be more transparent.

Senator Mitt Romney, the 2012 Republican presidential nominee, warned that evidence of Trump asking Ukraine's president to investigate Biden "would be troubling in the extreme."

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

Senator Marco Rubio said "no" when asked whether it was appropriate for Trump to discuss rival Biden on a call with Ukraine's president.

"But that's different from being an impeachable offense," he told AFP.

All eyes will be on Washington 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 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, 137 of them, plus Republican-turned-independent Justin Amash, support launching an impeachment inquiry, according to CNN's count.afp