Acting U.S. Ambassador to Ukraine Bill Taylor testified unequivocally Tuesday that President Trump pushed Ukraine to investigate both election interference and a company linked to former Vice President Joe Biden's son -- and was willing to hold up military aid and a White House meeting to get a public announcement from the country that the probes were underway.

In his opening remarks to House lawmakers obtained by Fox News, Taylor voiced his apparent frustration that the Trump administration was undercutting his personal policy preference for providing robust aid to Ukraine.

Among Taylor's colorful claims were that then-national security adviser John Bolton furiously warned that a Trump phone call with Ukraine's leader would be a "disaster," and that Taylor nearly didn't take the job leading Ukraine's embassy out of concerns the U.S. wouldn't be sufficiently helpful to Ukraine.

Republicans, however, have countered that military aid to Ukraine was released in September, and that there has been no evidence Ukrainians were aware that the aid was being withheld as part of any implicit quid pro quo. However, Politico reported that the Ukraine aid was being held up in August -- two weeks before it was released.

Ukrainian officials have denied that there was any undue pressure from the White House.

READ TAYLOR'S FULL OPENING STATEMENT

Taylor went on to describe the existence of an "irregular" communications channel with Ukraine led by Trump attorney Rudy Giuliani, and a "weird combination of encouraging, confusing, and ultimately alarming circumstances" once he arrived in Kiev. The statement confirmed previous reporting of Taylor's remarks by Fox News.

"During this same phone call I had with [National Security Council aide Tim] Morrison, he went on to describe a conversation [United States E.U.] Ambassador [Gordon] Sondland had with Mr. Yermak at Warsaw," Taylor testified, referring to a July 28 conversation. "Ambassador Sondland told [top Ukraine aide Andriy] Yermak that security assistance money would not come until President [Volodymyr] Zelenskyy committed to pursue the Burisma investigation."

Burisma Holdings is the Ukrainian natural gas company where Biden’s son Hunter was employed in a lucrative role despite no relevant expertise.

Taylor continued: "I was alarmed by what Mr. Morrison told me about the Sondland-Yermak conversation. This was the first time I had heard that the security assistance not just the White House meeting — was conditioned on the investigations."

That same day, Taylor said, he sent Sondland a text message asking if security assistance and a White House meeting "are conditioned on investigations," prompting Sondland to request Taylor call him. Although those texts have previously been released, the contents of Taylor's call have been unclear.

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"During that phone call, Ambassador Sondland told me that President Trump had told him that he wants President Zelenskyy to state publicly that Ukrain will investigate Burisma and alleged Ukrainian interference in the 2016 U.S. election," Taylor testified.

"Once I arrived in Kiev, I discovered a weird combination ofencouraging, confusing, andultimately alarming circumstances." — Acting U.S. Ambassador to Ukraine Bill Taylor

“By mid-July it was becoming clear to me that the meeting President Zelenskyy wanted was conditioned on the investigations of Burisma and alleged Ukrainian interference in the 2016 U.S. elections,” Taylor testified.

Also in his opening statement, Taylor described his commitment to providing support to Ukraine as so strong that he nearly threatened not to accept Secretary of State Mike Pompeo's offer for him to lead the U.S. Embassy in Kiev. Taylor, a retired diplomat, had been chosen to run the embassy after the administration abruptly ousted Ambassador Marie Yovanovitch.

Taylor had served as U.S. Ambassador to Ukraine from 2006 to 2009, but because he was not yet reconfirmed by the Senate, his official title was to be Chargé d 'Affaires ad interim. A former Army officer, Taylor had been serving as executive vice president at the U.S. Institute of Peace — which has described itself as a nonpartisan think tank founded by Congress — when he was appointed to run the embassy.

"I could be effective only if the U.S. policy of strong support for Ukraine, strong diplomatic support along with robust security, economic, and technical assistance — were to continue and if I had the backing of the secretary of state to implement that policy," Taylor said.

"During my meeting with Secretary Pompeo on May 28, I made clear to him and the others present that if U.S. policy toward Ukraine changed, he would not want me posted there and I could not stay," Taylor continued. "He assured me that the policy of strong support for Ukraine would continue and that he would support me in defending that policy."

The White House, meanwhile, fired back Tuesday over Taylor's testimony: "President Trump has done nothing wrong — this is a coordinated smear campaign from far-left lawmakers and radical unelected bureaucrats waging war on the Constitution. There was no quid pro quo," White House press secretary Stephanie Grisham said.

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Only Taylor's opening statement has been released at this point. However, lawmakers emerging after hours of the private deposition said Taylor relayed a "disturbing" account, including establishing a "direct line" to the quid pro quo at the center of Democrats' impeachment inquiry.

Lawmakers said Taylor recalled events that filled in gaps from the testimony of other witnesses, particularly Sondland, who testified last week and whose statements now are being called into question by Taylor's account. They said Taylor kept records of conversations and documents.

"The testimony is very disturbing," said Rep. Carolyn Maloney, D-N.Y. Rep. Dean Phillips, D-Minn., used the same word. Asked why, he said, "Because it's becoming more distinct."

Taylor's appearance was among the most-watched because of a text message, released by House investigators earlier in the probe, in which he called Trump's attempt to hold back military aid to Ukraine "crazy."

The account called into question the testimony from Sondland, a wealthy businessman who donated $1 million to Trump's inauguration, who told Congress last week he did not fully remember some details of the events. Sondland may be asked to return to Congress after he testified that, among other things, he was initially unaware that the gas company was tied to the Bidens.

Rep. Ami Bera, D-Calif., said Taylor, a career civil servant, had a better recall of details than Sondland.

Fox News has learned that House investigators have started to winnow the list of witnesses coming in for closed-door interviews or depositions and that the parade of witnesses expected for interviews could start to slow next week or the week after.

That’s because the "universe" of witnesses knowledgable about a possible quid pro quo was limited. Democrats also wanted to narrow their probe to Ukraine and not let it stray too far afield, Fox News learned.

Acting Assistant Secretary of European and Eurasian Affairs Philip Reeker is scheduled for a closed-door interview Saturday morning. That has been delayed due to funeral services for the late Rep. Elijah Cummings, D-Md. Fox News is told the odds of Reeker showing up are "50-50."

There also have been questions as to whether there will be more than a smattering of members coming to the Saturday meeting because of other commitments. Fox News is told members believed staff should be able to handle Reeker’s deposition because he's not a primary witness.

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"There's a hierarchy," one source close to the interviews said. "Reeker's like the sprinkles on the cake."

When asked about the importance of Reeker, another source told Fox News, "There’s a reason [he's] scheduled for Saturday."

Fox News' Kellianne Jones, Chad Pergram, Mike Emanuel, Alex Pappas and The Associated Press contributed to this report.