WASHINGTON (AFP) - US President Donald Trump, by withholding military aid from Ukraine to demand dirt on a potential 2020 election rival, went beyond anything done by disgraced predecessor Richard Nixon, the Democratic chair of the House impeachment panel alleged on Thursday (Nov 21).

"What we've seen here is far more serious than a third-rate burglary of the Democratic headquarters," House Intelligence Committee chairman Adam Schiff said in a reference to the Watergate scandal that led to Nixon's resignation.

"What we're talking about here is the withholding of recognition (for Ukrainian President Volodymyr Zelenskiy) in that White House meeting, the withholding of military aid to an ally at war," Schiff said.

"This is beyond anything Nixon did."

Trump is threatened with impeachment by the Democratic-controlled House of Representatives for pushing Zelenskiy in a July 25 phone call to investigate Democrat Joe Biden and his son, Hunter, who served on the board of a Ukrainian gas company.

Nixon resigned in 1974, two years after the burglary of Democratic offices in the Watergate complex in Washington.

Schiff's comments came at the end of five days of public hearings in the impeachment inquiry, which saw several witnesses testify that military aid to Ukraine was made contingent on Trump's demand that Zelenskiy announce an investigation of the Bidens.

"People look to us from all over the world," Schiff said.

"People in Ukraine who want a better future, they look to us.

"They still look to us and increasingly they don't recognise what they see," the Democrat from California said.

"That is a terrible tragedy for us but it is a greater tragedy for the rest of the world.

"In my view there is nothing more dangerous than an unethical president who believes they are above the law," Schiff said.