The State Department is blocking US Ambassador Gordon Sondland from appearing for scheduled Tuesday-morning testimony before Congress.

Sondland's attorney released a statement saying the State Department had "directed" the US ambassador to the European Union to skip his scheduled, voluntary testimony before three congressional committees pursuing an impeachment inquiry against President Donald Trump.

Sondland was one of several US diplomats who sent a series of text messages Congress released last week discussing Trump's military aid to Ukraine and Trump's conversations with Ukrainian President Volodymyr Zelensky.

The Wall Street Journal also reported that Sondland spoke with Trump during a five-hour gap between his texts to US diplomats.

After he spoke with Trump, he texted them back that the president had been "crystal clear" that there was no quid pro quo involved in Trump withholding military aid and his request for Ukraine to investigate a political rival.

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The State Department is blocking US Ambassador to the European Union Gordon Sondland — an important witness in the impeachment inquiry into President Donald Trump — from appearing for scheduled Tuesday-morning testimony before Congress.

Sondland's attorney Robert Luskin released a statement saying the State Department had "directed" Sondland to skip his scheduled, voluntary testimony before three congressional committees pursuing the impeachment inquiry.

"As a sitting US ambassador to the EU and employee of the State Department, Ambassador Sondland is required to follow the Department's direction," the statement said, adding that Sondland was "profoundly disappointed" that he was not able to testify on Tuesday and hoped "the issues raised by the State Department" would be resolved.

Trump also appeared to take some responsibility for blocking Sondland's testimony, saying in a series of tweets Tuesday that he believed the committees conducting the impeachment inquiry were a "totally compromised kangaroo court."

Read more: Meet the latest actor in the whirlwind Ukraine inquiry: hotelier turned ambassador Gordon Sondland

Sondland is one of several top US diplomats embroiled in the Ukraine controversy. At the heart of the controversy is a July 25 phone call Trump had with Ukrainian President Volodymyr Zelensky during which Trump repeatedly pressured Zelensky to investigate former Vice President Joe Biden and his son Hunter Biden — and possibly using military aid as a bargaining chip.

The impeachment inquiry began after an explosive whistleblower complaint filed by an anonymous intelligence community official in August claimed Trump was "using the power of his office to solicit interference from a foreign country in the 2020 U.S. election" in the call with Zelensky — just days after the White House withheld a $400 million military aid package from Ukraine.

A memo released by the White House summarizing the call showed that after telling Zelensky that "we do a lot" for Ukraine, Trump asked him for "a favor, though," by investigating both Hunter Biden's business dealings and CrowdStrike, the cybersecurity firm once hired by the Democratic National Committee.

The complaint mentions Sondland as well as Kurt Volker, the former US special representative to Ukraine, as being involved in the matter. This was confirmed when Congress released text messages between Volker, Sondland, and the US's chargé d'affaires in Ukraine, Bill Taylor, last week.

In one exchange that took place on July 21, Taylor texted Sondland saying Zelensky was "sensitive about Ukraine being taken seriously, not merely an instrument in Washington domestic, reelection politics."

Sondland replied: "Absolutely, but we need to get the conversation started and the relationship built, irrespective of the pretext. I am worried about the alternative."

Taylor wrote to Sondland in a September 1 exchange, "Are we now saying that security assistance and [White House] meeting are conditioned on investigations?"

Later, on September 9, Taylor wrote in a text message dated at 12:47 a.m., "As I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign."

Sondland responded five hours later, at 5:19 a.m.: "Bill, I believe you are incorrect about President Trump's intentions. The president has been crystal clear no quid pro quo's 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," Sondland added. "I suggest we stop the back and forth by text. If you still have concerns, I recommend you give Lisa Kennna or S a call."

Read more: Newly revealed text messages show Trump diplomats' internal turmoil over his pressure on Ukraine

The texts indicate that the diplomat's conversations weren't limited to text messages — making Sondland's testimony even more important to fill in the gaps.

The Wall Street Journal also reported that Sondland spoke with Trump on the phone at some point in the five hours before he responded to Taylor.

Secretary of State Mike Pompeo — Trump's most trusted official on foreign policy — was on the Trump-Zelensky call. Even before the State Department blocked Sondland's testimony, Pompeo was already accused of obstructing the congressional investigation into the matter.

In September, Pompeo sent a letter to the House Intelligence, Foreign Affairs, and Oversight committees, telling them their requests to depose department officials connected to the whistleblower's complaint were "not feasible" and "do not provide adequate time for the Department and its employees to prepare" for testimony.

In turn, Democratic lawmakers accused Pompeo of "intimidating Department witnesses" from testifying and warned that it "will constitute evidence of obstruction of the impeachment inquiry."

In a Tuesday-morning press conference, the House Intelligence Committee's chairman, Rep. Adam Schiff of California, said Sondland had additional documents and communications the State Department was withholding from Congress.

"The failure to produce this witness and the failure to produce those documents we consider strong evidence of obstruction of the constitutional functions of Congress," Schiff said.