A redacted version of a whistleblower complaint at the centre of an impeachment probe against U.S. President Donald Trump was released Thursday, and alleges Trump abused his office's power during a phone call with Ukraine's president.

The nine-page document was released ahead of testimony to House investigators from Joseph Maguire, the acting director of national intelligence, who had released the complaint to Congress and the public this week after weeks of delay.

The still-unnamed whistleblower, who filed the complaint Aug. 12, said they were reporting an "urgent concern" regarding Trump's behaviour while on a phone call this summer with Ukrainian President Volodymyr Zelensky.

"I am deeply concerned that the actions described below constitute 'a serious or flagrant problem, abuse, or violation of law or executive order' that 'does not include differences of opinion concerning public policy matters,' consistent with the definition of an 'urgent concern,'" the nine-page redacted report said, quoting the whistleblower in part.

The concerns cite Trump's July 25 call with Zelensky in which Trump prodded the Ukrainian president to investigate Democratic political rival Joe Biden, as being part of "ongoing concerns."

"In the days following the phone call, I learned from multiple U.S. officials that senior White House officials had intervened to `lock down' all the records of the phone call, especially the official word-for-word transcript of the call that was produced as is customary by the White House situation room," the complaint said.

The whistleblower's statement also raises concerns about the role of Trump's lawyer, Rudy Giuliani, in seeking assistance from Ukraine to benefit the president's 2020 re-election campaign.

It suggests Ukrainian leadership was "led to believe" that the phone call between Trump and Zelensky was conditional to whether Zelensky "showed willingness to "play ball" on issues raised by Giuliani.

Giuliani had publicly stated his intention to secure from Ukraine derogatory information about Biden and his son Hunter.

The complaint also details concerns from U.S. officials about "Giuliani's circumvention of national security decision-making processes."

'This is a coverup'

On Thursday, U.S. House Speaker Nancy Pelosi accused Trump of trying to cover up the attempt he made to pressure Ukraine into investigating Biden.

The leading Democrat in Congress, Pelosi told reporters that "This is a coverup. This is a coverup."

U.S. House Speaker Nancy Pelosi said Thursday that Trump tried to cover up an attempt he made to pressure Ukraine into investigating Biden. (Al Drago/Reuters)

Pelosi said that a House of Representatives impeachment inquiry launched this week would focus narrowly on the Ukraine episode and that other instances in which the Republican Trump may have abused the power of his office would be considered later.

She said there was no timeline for the inquiry.

"The president has been engaged in a coverup all along," Pelosi said, citing the whistleblower's complaint that she said outlined a White House effort to "lock down" all records of the call.

The Trump administration initially argued it was not covered by a whistleblower statute that would require its automatic disclosure to Congress. But the administration dropped its opposition to its release after a political firestorm erupted over news reports of some of the details in the complaint.

Pelosi cited the complaint as reporting "repeated abuse of an electronic records system designed to store classified, sensitive, national security information, which the White House used to hide information of a political nature."

Trump attacks report

Trump quickly denounced the report, tweeting only minutes after its release saying, "THE DEMOCRATS ARE TRYING TO DESTROY THE REPUBLICAN PARTY AND ALL THAT IT STANDS FOR." He also tweeted, "STICK TOGETHER, PLAY THEIR GAME, AND FIGHT HARD REPUBLICANS. OUR COUNTRY IS AT STAKE!"

The tweet disappeared from Trump's Twitter feed just before 1 p.m ET.

Speaking at a private fundraising event in New York after the complaint was released, Trump said whoever helped the whistleblower was almost a "spy."

"I want to know, who's the person who gave the whistleblower the information, because that's close to a spy," Trump said. "You know what we used to do in the old days when we were smart, with spies and treason, right? We used to handle it a little differently than we do now."

Sen. Lindsey Graham, a Republican from South Carolina, said Trump "should be allowed to know who these people are, because they're talking about impeaching him based on second-hand information."

This is a screenshot of the tweet that disappeared from Donald Trump's Twitter feed just before 1 p.m ET.

The White House claims the complaint "shows nothing improper."

Press secretary Stephanie Grisham argued the complaint "is nothing more than a collection of third-hand accounts of events and cobbled-together press clippings."

Grisham said Trump released a rough transcript of his call with Zelensky on Wednesday "because he has nothing to hide."

She said, "The White House will continue to push back on the hysteria and false narratives being peddled by Democrats and many in the mainstream media."

However, House Democrats who have read the document say it's "deeply disturbing."

In opening Thursday's House intelligence committee session, Chair Adam Schiff said Trump's actions as detailed read like "a classic organized crime shakedown."

Schiff said the whistleblower shows "more of a dedication of country, more of an understanding of the president's oath of office, than the president himself."