A senior US intelligence official was appearing before a congressional committee on Thursday to discuss a whistleblower complaint which The Washington Post said involves behaviour by President Donald Trump.
According to the newspaper, the complaint filed by a US intelligence official stemmed from Trump's communications with a foreign leader and a "promise" allegedly made by the president.
The foreign leader involved was not identified by the Post.
The intelligence official filed a complaint with Michael Atkinson, the inspector general (IC IG) of the intelligence community, the Post said, adding that Atkinson considered it a matter of "urgent concern" that necessitated the notification of congressional oversight committees.
Representative Adam Schiff, the Democratic chairman of the House Intelligence Committee, said Atkinson was appearing behind closed doors on Thursday for a briefing on his handling of the whistleblower complaint.
"The IC IG determined that this complaint is both credible and urgent, and that it should be transmitted to Congress under the clear letter of the law," Schiff said in a statement.
"The committee places the highest importance on the protection of whistleblowers and their complaints to Congress."