Secretary of State Mike Pompeo hit back Wednesday at House Democrats amid a standoff over their demands to hear from department employees in the Ukraine controversy, saying it is “fundamentally not true” that he is trying to block officials from testifying before their committees while maintaining he will not “tolerate” congressional bullying.

During a press conference in Rome early Wednesday, Pompeo also acknowledged for the first time that he was on President Trump’s July 25 phone call with Ukrainian President Volodymyr Zelensky -- the incident that has since touched off an impeachment inquiry. Trump is under fire for urging Zelensky, on that call, to investigate the Biden family over their dealings in the country. Pompeo on Wednesday described his involvement on that call as appropriate, and within the purview of his role as secretary of state.

But as Pompeo defended the administration's interaction with Ukraine, he also escalated his criticism of how House Intelligence Committee Chairman Adam Schiff, Foreign Affairs Committee Chairman Eliot Engel and Oversight Committee Chairman Elijah Cummings have handled their effort to set depositions with several State Department employees. Namely, he complained that they pressured the officials not to contact internal lawyers.

"What we objected to was the demands that were put that deeply violate fundamental principles of the separation of powers," Pompeo said. "They contacted State Department employees directly and told them not to contact legal counsel at the State Department, at least that has been reported to us."

Pompeo added that Democrats said representatives from the State Department were not able to be present during those depositions, which he said was not acceptable.

"So, the response that I provided to them was one that acknowledged that we will, of course, do our constitutional duty to cooperate with the co-equal branch but we are going to do so in a way that is consistent with the fundamental values of the American system," Pompeo said, adding: "We won't tolerate folks on Capitol Hill bullying and intimidating ... State Department employees. That is unacceptable and it's not something I am going to permit to happen."

Pompeo’s comments came after he first sent a letter to Democrats Tuesday saying their “requested dates for depositions are not feasible,” claiming the lawmakers had not given State Department employees enough time to prepare and voicing concern that the committee was trying to prevent State Department counsel from participating.

By late Tuesday, Schiff, D-Calif., Engel, D-N.Y., and Cummings, D-Md., told the State Department, in a fiery letter, that Pompeo may now be considered a “fact witness” in their formal and ongoing impeachment inquiry. The Democratic chairmen said Pompeo “now appears to have an obvious conflict of interest” because he “reportedly participated” in Trump’s phone call with Zelensky.

On Wednesday, Pompeo acknowledged he was on that call but said U.S. policy with Ukraine has been consistently focused on helping to get rid of corruption and to help the new administration build a “successful and thriving” country.

"I was on the phone call but it was in the context of now, I guess, I had been the secretary of state for coming on a year and a half," Pompeo said. "I know precisely what the American policy is with respect to Ukraine. It has been remarkably consistent and we will continue to drive those set of outcomes."

Pompeo added that the U.S. is focused on "taking down the threat that Russia posed to Ukraine," helping to "get corruption outside of their government," and helping the new government in Ukraine to "build a successful, thriving economy."

POMPEO ACCUSES DEMS OF 'BULLYING' STATE DEPT OFFICIALS WITH DEPOSITION DEMANDS

"It's what State Department officials that I had the privilege to lead have been engaged in and it is what we will continue to do even as this noise is going on," he said.

Amid the dispute with Congress, Democrats have set closed-door depositions with two officials -- scheduling former special envoy to Ukraine Kurt Volker for Thursday and ousted U.S. Ambassador Marie Yovanovitch for next week.

Meanwhile, Democrats said in their letter that Pompeo “should not be making any decisions regarding witness testimony or document production in order to protect himself or the president.”

“Any effort by the secretary of the department to intimidate or prevent witnesses from testifying or withhold documents from the committees shall constitute evidence of obstruction of the impeachment inquiry,” they wrote.

Democrats launched their formal impeachment inquiry last week after revelations that Trump sought an investigation by Zelensky and Ukrainian officials into former Vice President Joe Biden, his son Hunter and the Bidens' business activity in Ukraine. Joe Biden has acknowledged on camera that in spring 2016, when he was vice president and spearheading the Obama administration's Ukraine policy, he successfully pressured Ukraine to fire top prosecutor Viktor Shokin. At the time, Shokin was investigating Burisma Holdings — where Hunter had a lucrative role on the board despite limited relevant expertise. Biden allies maintain his intervention was driven by corruption concerns.

As part of their latest inquiry, Schiff, Engel, and Cummings subpoenaed Pompeo last week for documents related to Ukraine. Pompeo said on Tuesday that the State Department “intends to respond to that subpoena by the noticed return date” which is this Friday.

Fox News also has confirmed that State Department Inspector General Steve Linick will come to Capitol Hill on Wednesday to meet with House and Senate Committee aides regarding Ukraine.

The Democrats have demanded a list of State Department officials who might have been involved with the Ukraine conversation, according to the letter. The chairmen additionally requested any State Department records about Rudy Giuliani, Trump's personal attorney, and any records relating to U.S. military aid to Ukraine.

Fox News' Gregg Re and Alex Pappas contributed to this report.