Donald Trump's comments came on the day he was forced to release Ukraine call transcript

US President Donald Trump on Wednesday called the grounds for his impeachment laid out by Democrats "a joke," after he was accused of pressuring his Ukrainian counterpart for domestic political reasons.

"When they look at the information, it's a joke. Impeachment for that?" Trump told a news conference in New York, after House Speaker Nancy Pelosi announced the launch of impeachment proceedings Tuesday.

Trump also promised transparency over a whistleblower whose complaints triggered the probe into whether the US leader urged Ukraine's Volodymyr Zelensky to investigate rival Joe Biden.

"I've informed them -- all of the House members -- that I fully support transparency on the so-called whistleblower information, even though it was supposedly second-hand information," Trump said.

Trump's comments came on the day he was forced to release a call transcript showing a conversation he had with Zelensky on July 25.

The summary -- which is not a verbatim transcript -- shows Trump saying US Attorney General Bill Barr and the president's personal lawyer Rudy Giuliani would be in touch about probing Ukraine-related business activities of Biden and his son Hunter.

As Barack Obama's vice president, Biden and other Western leaders pressured Ukraine to get rid of the country's top prosecutor, Viktor Shokin, because he was seen as not tough enough on corruption.

The transcript did not show Trump explicitly tying aid to Ukraine to Zelensky probing Biden, but the two did discuss a major aid package, which had been frozen by Trump at the time and was only released in recent weeks.

Trump described the scandal surrounding the call as a "hoax" and a "witch hunt."

"I didn't do it, I didn't threaten anybody. No push, no pressure, nothing," Trump told reporters.

Speaking alongside Trump earlier, Zelensky insisted he had not been "pushed" by Trump during the controversial call.