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(CNN) —  

The State Department’s inspector general on Tuesday requested an urgent briefing with senior congressional staff members after Secretary of State Mike Pompeo pushed back on House Democratic demands to turn over documents related to Ukraine and to depose current and former State officials, according to sources briefed on the matter.

State inspector general Steve Linick plans to “provide staff with copies of documents related to the State Department and Ukraine” Wednesday afternoon in a bipartisan closed-door briefing with relevant congressional committees, a person familiar with the briefing told CNN.

The briefing comes amid the House Democrats’ impeachment investigation, which has been fueled by the US Intelligence Community’s inspector general’s review of a complaint by a whistleblower who alleged President Donald Trump sought help from Ukraine’s government to interfere in the 2020 elections.

One congressional aide described the State inspector general’s request as “highly unusual and cryptically worded.”

The inspector general said the reason for the briefing was the office had obtained documents from acting legal adviser in the State Department. The briefing comes as the House committees investigating Trump and Ukraine have delayed one of those depositions planned for this week, according to an aide, but former US special envoy to Ukraine Kurt Volker will appear Thursday.

The aide said Tuesday that the testimony of former US Ambassador to Ukraine Marie Yovanovitch, which had been scheduled for Wednesday, would now occur next Friday, following an agreement between both the committee’s and the former ambassador’s counsel.

Three committees — the House Foreign Affairs, Intelligence and Oversight panels — have scheduled the depositions as part of their probe into whether the President solicited help from a foreign government to dig up dirt on his political opponent, after a whistleblower filed a complaint about the President’s July 25 call with Ukrainian President Volodymyr Zelensky and an alleged cover up. The Intelligence Committee will also meet with Intelligence Community Inspector General Michael Atkinson on Friday for a closed briefing.

Yovanovitch and Volker were two of the five depositions that the committees have scheduled during the next two weeks while Congress is on recess. But on Tuesday, Pompeo accused the Democrats of “bullying” the State Department employees it wanted to testify, and said in a letter to House Foreign Affairs Chairman Eliot Engel that the committee’s schedule for depositions was not feasible and did not provide “adequate time for the Department and its employees to prepare.”

Volker, however, is no longer a State Department official after he resigned last week following the release of the whistleblower complaint and transcript of the President’s call.

The three House Democratic chairmen responded to Pompeo’s letter Tuesday with a statement charging that any effort to prevent the officials from speaking to Congress “is illegal and will constitute evidence of obstruction of the impeachment inquiry.”

Four of the five officials scheduled for depositions — Yovanovitch, Volker, Counselor T. Ulrich Brechbuhl and Ambassador Gordon Sondland – were mentioned in the whistleblower complaint on Ukraine. The fifth, Deputy Assistant Secretary George Kent, has overseen policy on Ukraine at the State Department since September 2018.

It’s not yet clear whether the other State Department officials will appear for the scheduled depositions. The congressional committees also issued a subpoena Friday to Pompeo for documents related to the Ukrainian investigation.

A rough transcript of a July phone call between Trump and Zelensky released last week showed Trump pushed for Zelensky to investigate former Vice President Joe Biden, and his son, Hunter. There is no evidence of wrongdoing by either Joe or Hunter Biden.

Volker was named in the whistleblower complaint, which said that a day after Trump’s call, Volker and Sondland met with Zelensky and provided advice about how to “navigate” Trump’s demands.

Volker also put Ukrainian Presidential Adviser Andriy Yermak in contact with the President’s personal lawyer, Rudy Giuliani, which a State Department spokesperson confirmed in August.

This story has been updated with additional developments Tuesday.

CORRECTION: This story has been updated to correctly reflect that an aide said on Tuesday that the testimony of former US Ambassador to Ukraine Marie Yovanovitch would be delayed.

CNN’s Lauren Fox and Zachary Cohen contributed to this report.