A key witness in the House impeachment inquiry was ordered not to appear at a scheduled deposition on Tuesday, ramping up tensions between congressional Democrats and the Trump administration.

The State Department instructed Ambassador to the European Union Gordon Sondland not to appear for the deposition, according to his counsel.

Sondland's lawyer said his client was "profoundly disappointed" that the State Department did not allow him to testify and expressed hope that the issues raised by the agency would be resolved promptly to allow him to testify. 

"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," Robert Luskin, Sondland's lawyer, said in a statement. 

As a sitting U.S. ambassador, Luskin said his client had no choice but to follow the State Department's direction. He noted that Sondland had traveled to Washington from Brussels to appear. 

Kurt VolkerKurt VolkerThe Hill's Morning Report - Bipartisan uproar over Trump's Syria move Democrats set to grill ambassador embroiled in Ukraine controversy A dozen House Democrats call on EU ambassador to resign amid Ukraine scandal MORE, the former special envoy to Ukraine, testified before the House Intelligence, Foreign Affairs, and Oversight and Reform committees last week and provided text messages between himself, Sondland and William Taylor, a top official in the U.S. Embassy in Ukraine.

Sondland is shown in the text messages to be rejecting concerns from Taylor, a career diplomat, that Trump wanted the Ukrainian government to dig up dirt on the Bidens to help his reelection campaign. Sondland is also shown suggesting that the diplomats take their conversations offline.

"As I said on the phone, I think it's crazy to withhold assistance for help with a political campaign," Taylor wrote in a Sept. 9 message.

Sondland replied: "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. I suggest we stop the back and forth by text," Sondland wrote.

Schiff said Tuesday morning that the State Department has not given Congress Sondland's personal device with pertinent messages.

"Not only is the Congress being deprived of his testimony, and the American people are being deprived of his testimony today, but we are also aware that the ambassador has text messages or emails on a personal device which have been provided to the State Department," Schiff told reporters. "Although we have requested those from the ambassador and the State Department is withholding those messages as well. Those messages are also deeply relevant to this investigation and the impeachment inquiry."

"We understand the reason why the State Department decided not to have Ambassador Sondland here today. It's based on the unfair and partisan process that Mr. Schiff has been running," Jordan told reporters.

Sondland also appears in the intelligence community whistleblower complaint. He allegedly accompanied Volker to meetings in Kiev with Zelensky and other Ukrainian officials to provide advice on how to "navigate" Trump's demands and the different messages they were receiving from "official U.S. channels on the one hand and from [Trump's personal lawyer Rudy] Giuliani on the other."

While Sondland in the text messages said he believed Taylor was mistaken about Trump's intentions, the Wall Street Journal reported last week that Sen. Ron JohnsonRonald (Ron) Harold JohnsonFormer Reagan official rips Republicans for backing Trump: 'It's like the invasion of the body snatchers' Brennan: Republican senators 'running scared' of president because he 'comes after them with a vengeance' Sunday shows - Second whistleblower grabs spotlight MORE (R-Wis.) was told by Sondland that the Trump administration wanted the Ukrainian government to launch an investigation into the Bidens in exchange for the military aid. Johnson said that Trump denied it when he personally spoke to the president.

In another message days before the call between Trump and Zelensky on July 25, Taylor said that "Zelensky is sensitive about Ukraine being taken seriously, not merely as an instrument in Washington domestic, reelection politics."

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

The three House committees are also trying to secure depositions with other State Department officials, including George Kent, the State Department’s deputy assistant secretary in the Bureau of European and Eurasian Affairs, and State Department counselor T. Ulrich Brechbuhl.

At least one deposition is scheduled for this week. Marie Yovanovitch, who served as U.S. ambassador to Ukraine until May, when the Trump administration recalled her from the post, is set to testify in closed session on Friday.

Updated at 12:03 p.m.