WASHINGTON – House Democrats on Tuesday subpoenaed Gordon Sondland, a U.S. ambassador involved in the Trump administration's dealings with Ukraine, after the State Department blocked his appearance before their impeachment inquiry.

Sondland, President Donald Trump's ambassador to the European Union, was scheduled to appear before a trio of committees Tuesday morning to answer questions about his role in pushing Ukraine's president to investigate former Vice President Joe Biden. The subpoena demands that he appear before the panels on Oct. 16 and hand over key documents by Oct. 14.

Robert Luskin, Sondland's attorney, said the State Department directed him not to cooperate with the probe or appear before lawmakers.

"Early this morning, the U.S. Department of State directed Ambassador Gordon Sondland not to appear today for his scheduled transcribed interview before the U.S. House of Representatives Joint Committee," Luskin said.

Luskin said Sondland had agreed "to appear voluntarily ... in order to answer the committee’s questions on an expedited basis." But as a sitting U.S. ambassador, he is required to follow the State Department’s directive, Luskin said.

Democrats, in the subpoena, noted that Sondland was directed not to cooperate in a 12:30 a.m. voicemail — just hours before he was set to appear before the panels. His attorney informed lawmakers about two hours before his appearance.

"In light of Secretary Pompeo's direct intervention to block your appearance before our Committees, we are left with no choice but to compel your appearance at a deposition pursuant to the enclosed subpoena," chairmen of the three committees wrote to Sondland.

Trump defended the decision to prevent Sondland's testimony, saying the Democrats' inquiry is illegitimate.

"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," Trump tweeted Tuesday morning.

Rep. Adam Schiff, D-Calif., who leads the House Intelligence Committee, said the administration's move to block Sondland's testimony, as well as withhold relevant documents, is "strong evidence of obstruction" of justice.

"The ambassador has text messages or emails on a personal device, which have been provided to the State Department," Schiff said. "The State Department is withholding those messages, as well" as blocking Sondland's testimony.

More:Read text messages between diplomats on Trump, Ukraine president

Sondland traveled to Washington from his posting in Brussels, and the committee staff expected him to testify. Schiff said he had no indication before Tuesday morning that Sondland would be a "no-show." Luskin said Sondland still wants to cooperate.

"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," he said.

A State Department spokeswoman did not respond to a request for comment.

More:A visual timeline of the text messages in the Trump-Ukraine affair

The State Department's decision to block Sondland from speaking to House investigators escalates the showdown between Congress and the White House over Democrats' demands for information on Trump's dealings with Ukraine.

"This is obstruction," tweeted Rep. Don Beyer, D-Va. "Sondland is a key witness to the President’s attempts to seek campaign interference from Ukraine. The President is obviously terrified of what Sondland might tell Congress. What else is the President hiding from the American people? This will not end here."

Rep. Mark Pocan, D-Wis., threatened to withhold Secretary of State Mike Pompeo's salary over the State Department's decision to block Sondland's testimony.

“While Democrats seek to understand the full scope of the president’s criminal actions, we refuse to bankroll those in his administration who hold witnesses hostage,” Pocan said in a statement Tuesday.

Republicans argued Trump was right to block Sondland's appearance before the House Democrats' impeachment probe.

"It’s based on the unfair and partisan process that Mr. Schiff has been running," said Ohio Rep. Jim Jordan, the top Republican on the House Oversight Committee.

Last week, House Democrats issued a subpoena to the White House for documents relating to the Ukraine matter after Trump aides said the president would not cooperate with the impeachment inquiry until the full House votes to authorize it.

Before the announcement about the subpoena, Schiff said Tuesday it's "hard to overstate" Sondland's relevance to the impeachment inquiry, which is focused on allegations that Trump used the power of his office to solicit foreign interference in the 2020 election.

In a phone call July 25, Trump pressed Ukraine's president, Volodymyr Zelensky, to open an investigation into Biden and to probe allegations about Ukraine's role in the 2016 U.S. presidential election.

Before the Trump-Zelensky call, Sondland and Kurt Volker, who was Trump's special envoy to Ukraine, worked behind the scenes: Text messages show them pushing Ukrainian officials to make a public promise that they would investigate Biden and alleged 2016 election meddling. .

Democrats are investigating whether Trump withheld U.S. military aid to Ukraine until he secured those commitments and whether he used the promise of a White House meeting with Zelensky as further leverage.

"Heard from the White House," Volker wrote in a text to a top Zelensky adviser July 25, just before Trump and Zelensky were scheduled to speak by phone.

"Assuming President Z (Zelensky) convinces trump he will investigate/'get to the bottom of what happened' in 2016, we will nail down a date for visit to Washington. Good luck!" Volker told his Ukrainian counterpart.

After the Trump-Zelensky call, an unidentified whistleblower filed a complaint accusing Trump of using the power of his office to solicit foreign interference in the 2020 election. Biden is a leading 2020 candidate for the Democratic nomination.

Even before the whistleblower complaint, the top U.S. diplomat in Ukraine, Bill Taylor, raised concerns about the link between Trump calling for an investigation and granting Zelensky a meeting at the White House. Taylor pressed Volker and Sondland on whether Trump was withholding nearly $400 million in U.S. military aid until Zelensky delivered on his demands.

"Are we now saying that security assistance and WH meeting are conditioned on investigations?" Taylor asked in a text message to Volker and Sondland on Sept. 1.

"Call me," Sondland texted back.

Taylor responded: "As I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign."

Sondland said Trump had been "crystal clear no quid pro quo's of any kind ... I suggest we stop the back and forth by text." He told Taylor to call "S," presumably referring to Secretary of State Pompeo, if he wanted to discuss the matter further.

Sen. Ron Johnson, R-Wis., told The Wall Street Journal last week that Sondland told him Ukraine would appoint a prosecutor who would, as Johnson put it, work to "get to the bottom of what happened in 2016 – if President Trump has that confidence, then he’ll release the military spending."

“At that suggestion, I winced,” Johnson told the Journal. “My reaction was: Oh, God. I don’t want to see those two things combined.”

Johnson said he asked Trump about it and the president denied it.

"He said – expletive deleted – ‘No way. I would never do that. Who told you that?'” Johnson told the Journal.

Democrats said the texts prove a quid pro quo – Trump wanted investigations into Biden and the 2016 election before he would meet with Zelensky. Trump's supporters said there was no such condition and the call was perfectly appropriate.

Contributing: David Jackson and John Fritze