Representative Devin Nunes, a Republican from California and ranking member of the House Intelligence Committee, speaks during a hearing with Joseph Maguire, acting director of national intelligence, in Washington, D.C., U.S., on Thursday, Sept. 26, 2019.

The House Intelligence Committee's top Republican told acting intelligence chief Joseph Maguire to "be careful what you say" in his public hearing Thursday.

Rep. Devin Nunes, the committee's ranking member and one of President Donald Trump's most loyal allies in Congress, told Maguire that "we appreciate you being here and have fun" — but warned him to watch his words because the panel's Democrats are "going to use these words against you."

Maguire, who was appointed acting director of national intelligence in early August, responded: "Either way, I'm honored to be here and I'm honored to be leading the intelligence community."

A spokesman for Nunes did not immediately respond to CNBC's request for comment.

Maguire's testimony began less than two hours after the public release of a whistleblower's complaint about Trump's call with Ukraine president Volodymyr Zelensky in late July. Media reports about that call, in which Trump asks Zelensky "if you can look into" issues related to former Vice President Joe Biden and his son Hunter, led Democratic leaders to announce a formal impeachment inquiry into Trump earlier this week.

The complaint details an "urgent concern" that Trump is "using the power of his office to solicit interference from a foreign country in the 2020 U.S. election."