The House speaker, Nancy Pelosi, is narrowing the focus of the impeachment inquiry to focus on the Trump-Ukraine scandal, as the White House reels from one of the most tempestuous weeks of Donald Trump’s presidency.

The revelations that Trump pushed Ukraine’s president, Volodymyr Zelenskiy, to investigate a political rival – and a whistleblower’s accusation that the White House then attempted a cover-up – will now serve as the main thrust of Democrats’ investigation, according to reports.

In narrowing the investigation, from a wide-ranging examination of the lengthy Trump-Russia investigation, Pelosi is said to be hoping for a quicker resolution to the impeachment inquiry. The Washington Post, citing Democrats and congressional aides, said Pelosi has instructed House committees to file the results of their individual investigations within weeks.

That could build towards a vote around Thanksgiving, reports suggested. The celebration, always on the fourth Thursday in November, falls on 28 November this year.

Adam Schiff, the Democratic chairman of the House intelligence committee, said on Friday he will oversee hearings next week – a period which had originally been set aside as a congressional recess. Rudy Giuliani, the president’s personal lawyer, and William Barr, the attorney general, are set to be called to testify before the panel.

Democrats’ acceleration of their investigation came as Donald Trump sought to disparage both the whistleblower and those who provided the whistleblower with information.

“Sounding more and more like the so-called whistleblower isn’t a whistleblower at all.” Trump tweeted, providing no evidence for his claim.

“In addition, all second hand information that proved to be so inaccurate that there may not have even been somebody else, a leaker or spy, feeding it to him or her? A partisan operative?”

In the complaint, the whistleblower said their information came from national security officials with knowledge of Trump’s actions.

Despite Trump’s attempt to discredit the whistleblower, on Friday the White House confirmed a key aspect of the complaint. A senior White House official acknowledged to CNN that the Trump administration has previously used a separate classified system to house important documents, including the transcript of the president’s Ukraine call.

The attacks on the whistleblower’s credibility appeared to be a deliberate ploy, one Republicans are likely to return to as the investigation continues. Senior White House adviser Kellyanne Conway said it was a “stretch” to refer to the whistleblower as such, while Giuliani diminished the allegations and said that, in fact, he was a “legitimate whistleblower” in the saga.

The fallout from the whistleblower’s complaint and the release of the transcript of the Trump-Zelenskiy call continued on Friday. More than 300 former national security officials signed a statement condemning Trump’s actions, as reports suggested the White House knew of the whistleblower’s complaint more than a week before it was formally referred to the justice department.

“President Trump appears to have leveraged the authority and resources of the highest office in the land to invite additional foreign interference into our democratic processes,” the officials, who include people who have worked in both Republican and Democratic governments, wrote.

“That would constitute an unconscionable abuse of power.”

Trump continued to criticize the media and defend his discussion with Zelenskiy.

“I had a simple and very nice call with with the new President of Ukraine, it could not have been better or more honorable, and the Fake News Media and Democrats, working as a team, have fraudulently made it look bad. It wasn’t bad, it was very legal and very good. A continuing Witch Hunt!” the president tweeted.

The complaint by an unnamed whistleblower was released on Thursday. The whistleblower alleged that Trump’s actions on Ukraine “pose risks to US national security”. According to the complaint the White House intervened to “lock down” the transcript of the July call between Trump and Zelenskiy, in which Trump pressured Zelenskiy to investigate Joe Biden.

“In the course of my official duties, I have received information from multiple US government officials that the president of the United States is using the power of his office to solicit interference from a foreign country in the 2020 US election,” the whistleblower wrote.

The complaint was released as the acting director of national intelligence, Joseph Maguire, testified before the House intelligence committee. Maguire said he had initially been blocked by the White House from releasing the complaint to Congress.

In his opening remarks on Thursday Maguire said the situation was “totally unprecedented”, later saying the whistleblower had “done the right thing”.

The New York Times reported that the White House was alerted to the detail of the complaint a week before it was formally referred.

The lawyer shared the whistleblower’s allegations with the White House, following normal procedure, as the formal complaint moved up the intelligence community ladder. The news – confirmed by CNN reporting – that the White House had prior knowledge of the accusations in the complaint is likely to raise questions about how the Trump administration then sought to delay its release.

The complaint details how the White House sought to block access to the transcript of the call with Zelenskiy – in which Trump asked the Ukraine president to “do us a favor” and offered help in investigating Joe Biden, a potential 2020 presidential rival.

A number of different House committees – led by Democrats – were already investigating Trump for impeachable actions, based on special counsel Robert Mueller’s Trump-Russia investigation.

At a private event in New York on Thursday, the president lashed out at those who helped to inform the whistleblower and alluded to retaliation.

In audio obtained and released by the Los Angeles Times, Trump says: “Who’s the person that gave the whistleblower the information? Because that’s close to a spy. You know what we used to do in the old days, when we were smart, right? The spies and treason? We used to handle it a little differently than we do now.”

Pelosi accused the White House of a cover-up.

• This article was amended on 30 September 2019 because an earlier version was incorrect to say that Thanksgiving is always on the “last Thursday in November”. It is celebrated on the fourth Thursday of November which is not always the last Thursday in that month.