President Trump's former top assistant working on European and Russian issues will reportedly testify to Congress that Trump's top lawyer, Rudy Giuliani, and his associates set up a shadow foreign policy for the Trump administration relating to Ukraine that circumvented the National Security Council (NSC).

NBC News reported Thursday that Fiona Hill, a former special assistant to the president, will tell House lawmakers that Giuliani and Trump's ambassador to the European Union, Gordon Sondland, went around the NSC and official White House procedures to speak to the president about Ukraine.

Hill's attorney did not immediately return a request for comment from The Hill.

Her impending testimony is said to have caused worry in the White House, as Hill's tenure as one of Trump's top advisers stretches to the earliest months of his presidency.

Hill left the Trump administration in June after serving under two national security advisers, H.R. McMaster and John Bolton, the latter of whom was ousted from the Trump administration last month.

Considered by many to be a top expert on Russian President Vladimir Putin, Hill is the first former White House official to agree to comply with House Democrats' impeachment inquiry.

The probe was launched in response to a controversial conversation between the president and Ukraine's President Volodymyr Zelensky during which Trump pressed Zelensky to investigate former Vice President Joe Biden, a top 2020 Democratic contender for president.

The White House has opted to use executive privilege to stop officials from testifying before Congress, but it remains legally unclear if the president can force former officials who are now private citizens from testifying.