US President Donald Trump  (File photo: Xinhua)

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.

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

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.