President Trump lashed out at congressional Democrats on Saturday over their probe into his dealings with Ukraine ahead of the latest deposition in the impeachment inquiry.

"The Ukraine investigation is just as Corrupt and Fake as all of the other garbage that went on before it," Trump tweeted Saturday morning amid a series of attacks on political rivals.

Trump also pushed back on a Washington Post report that said he was frustrated with House Democrats' impeachment effort.

"I am not because I did nothing wrong," Trump tweeted, while reiterating his past criticisms of House Intelligence Committee Chairman Adam Schiff (D-Calif.), who is helping lead the impeachment probe.

Trump sent the tweets Saturday morning as a top State Department official with oversight of Ukraine arrived at the Capitol, where lawmakers are expected to press him on the Trump administration's pressure campaign on foreign leaders.

Philip Reeker, acting assistant secretary of European and Eurasian Affairs, entered the Capitol shortly after 10:30 a.m. and was ushered into the secure room where three House committees - Intelligence, Oversight and Reform, and Foreign Affairs - are conducting the impeachment inquiry into Trump's dealings with Ukraine.

Reeker walked silently past a bank of TV cameras and reporters, ignoring a question about whether he was subpoenaed or came voluntarily. An official working on the impeachment inquiry later said that Reeker had been subpoenaed.

Like other career officials' depositions before him, Democrats subpoenaed Reeker to testify amid White House efforts to block current and former administration officials from interviewing with House investigators.

"In light of an attempt by the State Department to direct Ambassador Philip Reeker not to appear for his scheduled deposition, and efforts by the State Department to also limit any testimony that does occur, the House Intelligence Committee issued a subpoena to compel his testimony," according to an official working on the impeachment inquiry. "As is required of him, Ambassador Reeker is now complying with the subpoena and answering questions."

George Kent, a top State Department official who testified earlier this month, brought his concerns to Reeker, one of his superiors, about a shadowy campaign to oust then-U.S. Ambassador to Ukraine Marie Yovanovitch, according to emails reportedly given to House investigators by the State Department inspector general.

Kent, the deputy assistant secretary of State, also related these concerns to David Hale, a top-ranking State Department official, and to T. Ulrich Brechbuhl, who is close to Secretary of State Michael Pompeo.

Democrats subpoenaed Brechbuhl as well as two other Trump administration officials for testimony in early November, after they declined to testify voluntarily.

Reeker is the ninth witness to be deposed since Speaker Nancy Pelosi (D-Calif.) announced the launch of a formal impeachment inquiry into Trump, which was prompted by allegations from a government whistleblower that Trump had sought help from Ukrainian President Volodymyr Zelensky to find dirt on political opponents - and threatened to withhold U.S. military aid to Kiev if Zelensky didn't comply.

The administration's campaign - spearheaded by Rudy Giuliani, Trump's personal lawyer - resulted in the removal of Yovanovitch in May, after she'd voiced concerns about the effort to press foreign leaders for political favors.

The rare Saturday session appears to be sparsely attended. A handful of lawmakers were seen entering the room - including Reps. Stephen Lynch (D-Mass.), Tom Malinowski (D-N.J.) and Mark Meadows (R-N.C.) - but lawmaker participation in the staff-led deposition seems to have dropped off relative to the interviews conducted earlier in the week, when Congress was in session.

Yovanovitch testified earlier this month, bluntly describing what she viewed as a shadowy effort by Trump and other officials to pressure the State Department for her removal.

"Although I understand that I served at the pleasure of the President, I was nevertheless incredulous that the U.S. government chose to remove an Ambassador based, as best as I can tell, on unfounded and false claims by people with clearly questionable motives," she testified before House investigators, according to her leaked opening remarks.

Yovanovitch, who detailed her decades of experience in foreign service, identified Giuliani and his associates as individuals who took part in the campaign against her.

"I do not know Mr. Giuliani's motives for attacking me. But individuals who have been named in the press as contacts of Mr. Giuliani may well have believed that their personal financial ambitions were stymied by our anti-corruption policy in Ukraine," her prepared remarks read.

The longtime civil servant said that at the time she learned of her firing, a superior at the State Department informed her that there was a "concerted campaign against" her and that "the Department had been under pressure from the President" to remove her since last summer. She strongly denied claims that she made disparaging remarks about Trump, which some critics of hers pointed out to Pompeo.