Tim Morrison, President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE's top Russia official on the National Security Council (NSC), is expected to soon leave his post, a senior administration official confirmed to The Hill on Wednesday.

“After more than a year of service at the National Security Council, Mr. Morrison has decided to pursue other opportunities – and has been considering doing so for some time. We wish him well," the official said.

NPR first reported the news.

Morrison is expected to testify to House committees Thursday morning as a part of House Democrats' ongoing impeachment inquiry.

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His replacement will be Andrew Peek, a deputy Assistant Secretary of State for Iraq and Iran in the Bureau of Near Eastern Affairs, according to NPR.

Morrison assumed his post in July after Fiona Hill announced that she would be vacating the position in August.

Morrison, known as a defense hawk, has a significant role in the impeachment proceedings.

The U.S.'s top diplomat to Ukraine, William Taylor, testified earlier this month to House investigators that Morrison made contact with NSC lawyers regarding the Trump administration allegedly demanding Ukrainian officials investigate former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE and his son, Hunter Biden.

In his testimony, Taylor said that Morrison told him that Gordon Sondland, the U.S. ambassador to the European Union, had informed an Ukrainian official that promised U.S. military aid would not be delivered unless Ukrainian President Volodymyr Zelensky publicly announced that he would investigate Hunter Biden.

It is unclear whether Morrison was asked to leave his position or not. Barbara Van Gelder, his lawyer, would not discuss his job status. NPR could not reach Peek for comment, although his out-of-office reply on his State Department email says that he's been assigned a position on the NSC.

A former NSC official told NPR, "If there was a quid pro quo, he would know."

"He would know if there is a hold on military assistance," the official added.

The Hill has reached out to the NSC for comment.

Morgan Chalfant contributed.