Gordon Sondland, the U.S. ambassador to the European Union, was directed by the State Department not to appear Tuesday for a scheduled interview with House committees leading the impeachment inquiry into President Donald Trump.

Sondland, a Trump political appointee, has emerged as a central player in Trump's bid to persuade Ukraine’s new government to commit publicly to investigate corruption and the president's political opponents.

"Ambassador Sondland had previously agreed to appear voluntarily today, without the need for a subpoena, in order to answer the Committee’s questions on an expedited basis," Robert Luskin, Sondland's attorney, said in a statement.

"As the sitting U.S. Ambassador to the E.U. and employee of the State Department, Ambassador Sondland is required to follow the Department’s direction," Luskin continued, adding that Sondland "is profoundly disappointed that he will not be able to testify today."

Luskin noted that Sondland traveled to Washington from Brussels "in order to prepare for his testimony and to be available to answer the Committee’s questions."

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"Arrangements had already been made with Joint Committee staff regarding the logistics of his testimony," Luskin said. "Ambassador Sondland believes strongly that he acted at all times in the best interests of the United States, and he stands ready to answer the Committee’s questions fully and truthfully."

Luskin said the ambassador "hopes" the State Department's qualms that "precludes his testimony will be resolved promptly."

"He stands ready to testify on short notice, whenever he is permitted to appear," Luskin said.

The New York Times was first to report that the administration had blocked Sondland's interview, a move that is certain to inflame tensions between the White House and the Democrats leading the impeachment inquiry.

Those Democrats, the chairmen of House Foreign Affairs, Intelligence and Oversight committees, subpoenaed Sondland as a result, seeking both his testimony and documents they charge Secretary Mike Pompeo is withholding from Congress. The chairmen gave Sondland until Oct. 14 to comply with the document request, and rescheduled his deposition for Oct. 16.

The subpoena was issued Tuesday evening, shortly after the White House sent a defiant letter to House Speaker Nancy Pelosi and the three chairmen saying it "cannot comply" with the impeachment inquiry and refusing to hand over internal documents regarding Ukraine. The move was the latest demonstration of a White House strategy of almost-universal resistance taking shape in its efforts to stymie the Democratic investigation into whether Trump used the power of his office to pressure Ukraine to conduct probes that could be politically advantageous.

By blocking the ambassador's testimony, "the White House has once again attempted to impede and obstruct the impeachment inquiry," the Democratic chairmen said in a joint statement earlier Tuesday.

According to the chairmen, the State Department left Sondland's attorneys a voicemail at 12:30 a.m. informing them that "the Trump Administration would not allow the Ambassador to appear today as part of the House’s impeachment inquiry."

“In addition, Ambassador Sondland’s attorneys have informed us that the Ambassador has recovered communications from his personal devices that the Committees requested prior to his interview today," the chairs wrote. "He has turned them over to the State Department, however, and the State Department is withholding them from the Committees, in defiance of our subpoena to Secretary Pompeo."

Sondland, a hotelier and Republican megadonor, was nominated to be ambassador to the European Union one year after he made a $1 million donation to Trump's inaugural committee. Text messages provided to Congress show Sondland and another ambassador worked to persuade Ukraine to publicly commit to investigating Trump’s political opponents and explicitly linked the inquiry to whether Ukrainian President Volodymyr Zelenskiy would be granted an official White House visit.

The messages, released last week by House Democrats, show the diplomats coordinating with both Trump's personal attorney Rudy Giuliani and a top Zelenskiy aide.

Trump tweeted about blocking Sondland's planned testimony Tuesday, writing, "I would love to send Ambassador Sondland, a really good man and great American, to testify, but unfortunately he would be testifying before a totally compromised kangaroo court, where Republican’s rights have been taken away, and true facts are not allowed out for the public to see."

He then appeared to confuse Sondland's text messages for tweets.

"Importantly, Ambassador Sondland’s tweet, which few report, stated, 'I believe you are incorrect about President Trump’s intentions. The President has been crystal clear: no quid pro quo’s of any kind.' That says it ALL!" Trump said on Twitter.

The exchange Trump refers to shows William Taylor, the top U.S. diplomat in Ukraine, seeking clarification from Sondland on conditions for the White House meeting and whether the administration was withholding almost $400 million in Congress-approved security aid to Ukraine in exchange for politically beneficial probes.

"Are we now saying that security assistance and WH meeting are conditioned on investigations?” Taylor texted Sondland in early September.

"Call me," Sondland responded.

One week later, Taylor texted Sondland: "As I said on the phone, I think it’s crazy to withhold security assistance for help with a political campaign."

Several hours later, Sondland pushed back, saying Trump "has been crystal clear no quid pro quos of any kind."

"The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelensky promised during his campaign," he continued. "I suggest we stop the back and forth by text."

Sondland consulted directly with Trump before sending that followup message, a person with knowledge of the call confirmed to NBC News. The Wall Street Journal was first to report on the conversation between Trump and Sondland.

More than a dozen House Democrats have called for Sondland to resign from his role in the Trump administration in light of revelations regarding his role in the Ukraine matter. Volker, who testified to Congress last week, resigned after being named in the intelligence community whistleblower complaint about Trump's July phone call with Zelenskiy.

"FYI the State Department doesn’t keep these people under lock and key," Sen. Chris Murphy, D-Conn., tweeted. "Gordon Sondland is an adult — he can obey the law and testify if he wants. We shouldn’t let him or anyone else get away with blaming their violation of the law" on Secretary of State Mike Pompeo.

According to the White House summary of that call, Trump asked Zelenskiy to look into former Vice President Joe Biden and his son, Hunter, as well as a conspiracy theory regarding Ukrainian meddling in the 2016 presidential election. The whistleblower’s complaint alleged that Volker went to Kyiv to try to guide Ukraine officials on how to handle Trump’s demands for them to investigate the younger Biden's nearly five years as a member of the board that manages Ukraine's Burisma, a natural gas producer.

Sen. Ron Johnson, R-Wisc., said in an interview with The Wall Street Journal published Friday that Sondland informed him the security aid would be released when Trump gained confidence that Ukraine would move to "get to the bottom of what happened in 2016."

Johnson, who told the paper that he’d reached out to Sondland about the delay in aid, which Johnson had advocated for, said the suggestion of a contingency arrangement made him "wince."

"My reaction was: 'Oh, God. I don’t want to see those two things combined,'" Johnson told the paper.

Johnson noted that he then called Trump, who denied the allegation.

Trump has said he froze military aid to Ukraine because he wanted European countries to provide more assistance. The administration froze the almost $400 million in aid one week prior to Trump's July phone call with Zelenskiy, and released it just before Democrats revealed the existence of the whistleblower complaint last month.

Speaking with reporters Tuesday, House Intelligence Committee Chairman Adam Schiff, D-Calif., lamented that "not only is the Congress being deprived of" Sondland's testimony, "but we are also aware the ambassador has text messages or emails on a personal device which have been provided to the State Department."

Schiff said the committee requested those messages, which he said the State Department is withholding. He called the messages "deeply relevant to this investigation and the impeachment inquiry" and said Democrats would consider the failure to obtain Sondland's documents and testimony as evidence of obstruction.

Rep. Jim Jordan, R-Ohio, the ranking member of the House Oversight Committee, told reporters Tuesday he wished Sondland could have testified, but said Republicans "fully understand why the administration made the decision it did," criticizing what he deemed as selective releases from the committee regarding Volker's testimony last week.

Asked if it was proper for Trump to ask Ukraine and China to investigate the Bidens, as Trump publicly called for last week, Jordan said, "The president is doing his job."