WASHINGTON, Sept. 25 (Xinhua) -- U.S. President Donald Trump said Wednesday House Democrats' decision to launch a formal impeachment inquiry into him was a "disgraceful thing," shortly after the White House released a memo of a phone conversation between the president and his Ukrainian counterpart.

Speaking to reporters in New York, Trump said the impeachment inquiry was "the single greatest witch hunt in American history."

"It's a disgraceful thing," the president said. "The letter was a great letter, meaning the letter revealing the call."

The White House earlier in the day released a rough transcript of the phone call on July 25 between Trump and Ukrainian President Volodymyr Zelensky.

The presidential conversation is under multiple congressional investigations, and is at the center of increasingly heightened tensions between Congress and the White House stemming from a whistleblower complaint filed in mid-August by an unidentified intelligence official, which alleged that Trump interacted inappropriately with a foreign leader and made an unspecified "promise."

According to the rough transcript, which the White House said in a disclaimer "is not a verbatim transcript of a conversation," Trump mentioned Democratic presidential candidate Joe Biden several times during the call, at one time suggesting that Zelensky cooperate with U.S. Attorney General William Barr in investigating into how Biden stopped a prosecution of a Ukrainian gas company which might be related to his son, Hunter Biden.

Trump, the transcript showed, also asked Zelensky to do the United States a "favor" by looking into a congressional testimony by Special Counsel Robert Mueller, who investigated whether Russia interfered into the 2016 U.S. election and whether Trump obstructed justice. The president said "a lot of" issues surrounding the testimony "started with Ukraine."

Trump told reporters in New York that the transcript suggested he put "no pressure" on Zelensky. "It turned out to be a nothing call."

House Speaker Nancy Pelosi, who announced the impeachment inquiry Tuesday, issued a statement regarding the transcript, stressing once again the necessity of the impeachment inquiry.

The transcript, Pelosi said, "confirms that the President engaged in behavior that undermines the integrity of our elections." She added that the transcript and the "Justice Department's acting in a rogue fashion in being complicit in the President's lawlessness confirm the need for an impeachment inquiry."