"We have to be fair to the president, and that's why it's an inquiry and not an outright impeachment. And we have to give the president the chance to exonerate himself," she told reporters in Washington.
"We want to be fair as we go forward," she added.
Trump spokeswoman Stephanie Grisham pushed back, tweeting while Pelosi was still talking that it is "laughable" to consider that the Democrats would treat the president fairly. "The Witch hunt continues," she said, echoing the president's language.
Trump has ramped up his rhetoric on the impeachment inquiry, labelling the investigation a "coup" designed to "take away the power of the people."
The remarks came as two State Department officials who worked on Ukraine agreed to testify this week and next before lawmakers. A key intelligence official is also due on Capitol Hill on Friday.
US Secretary of State Mike Pompeo, who is also facing a subpoena from Congress as part of the investigation, confirmed he took part in a key phone call Trump held with the Ukrainian president on July 25.
"I was on the phone call," Pompeo said on Wednesday during an official visit to Italy. Lawmakers have said his participation would make the secretary a witness in their investigations.
So far, Pompeo has declined to confirm he will comply with lawmakers' demands and bashed the inquiry as intimidation against his department, tapping into a repeated play by the White House to counterattack when under pressure.
Democrats in the US House of Representatives have focused their inquiry on Trump's call with Ukrainian President Volodymyr Zelensky in which the Republican president asked his counterpart to potentially investigate Joe Biden, a Democratic rival running in the 2020 race.
There are also questions as to whether Trump withheld military aid to Ukraine as part of pressure on that country, in defiance of Congress.
Lawmakers have said they are likely to issue a subpoena for documents directly to the White House on Friday, further raising the stakes.
"As I learn more and more each day, I am coming to the conclusion that what is taking place is not an impeachment, it is a COUP," Trump tweeted late Tuesday.
Trump has been using increasingly strong language to lash out at the Democrats, a whistleblower who sounded an alarm about the call and the inquiry itself.
Over the weekend, he tweeted that if he were to be removed from office through a full impeachment process, it could lead to "civil war," a remark that drew concern even from members of his own Republican Party.
On Monday, Trump dubbed the whistleblower whose complaint helped spark the impeachment inquiry a "fake" and also questioned whether Democrat Adam Schiff, the head of the House Intelligence Committee, should be arrested for treason.
"This is a blatant effort to intimidate witnesses. It's an incitement of violence," Schiff told reporters, standing next to Pelosi, referring to the president's language.
The remarks about the whistleblower, as well as a 50,000-dollar bounty for the person's identity put out by Trump supporters, has caused the person's lawyers to announce they fear for the safety of their client.
"The president probably doesn't realize how dangerous his statements are," Pelosi chided Trump.
The Democrats have promised to protect the anonymity of the whistleblower, saying this is enshrined in law. Trump has demanded the chance to meet the person.
"We take this to be a very sad time for the American people, for our country," Pelosi said of the impeachment investigations.
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