The president interacted with the unnamed leader in a phone call, according to one former intelligence official.
He made a “promise” during the interaction, according to two sources who spoke to the The Washington Post, which was considered so worrying that an official filed a complaint with Michael Atkinson, the intelligence community’s inspector general, on 12 August.
The 73-year-old spoke to at least five leaders in the five weeks before the concerns were raised.
He spoke to Vladimir Putin on 31 July and received two letters from Kim Jong Un over the summer. Mr Trump also met the emir of Qatar, the prime minister of Pakistan and the leader of the Netherlands in July.
Mr Atkinson found the complaint credible and that the matter was one of “urgent concern.”
The “urgent concern” standard is a US legal threshold.
Once this threshold has been passed, officials are required by law to notify congressional oversight committees about the complaint.
Mr Atkinson sent the “urgent concern” notification on 26 August.
Joseph Maguire, the acting director of national intelligence, has since refused to share details of the alleged promise with congress, as required by law.
He had until 2 September to do so.
“Contrary to your express obligations under the law, you are withholding...an authorised and protected whistleblower disclosure,” Adam Schiff, the chair of the House Intelligence Committee, said in a letter sent to Mr Maguire.
The Intelligence Community Whistleblower Protection Act 1998 (ICWPA) allows officials to report “urgent concerns” to congress “without suffering reprisals.”
Mr Schiff said whistleblower complaint involved “a serious or flagrant problem, abuse, violation of law or Executive Order, or deficiency relating to the funding, administration, or operation of an intelligence activity involving classified information”, as defined by the legislation.
“In an unprecedented departure from past practice, you have not transmitted the disclosure to the committee, not have you notified the committee of the fact of the disclosure of your decision not to transmit it to the committee,” he wrote.
“You have taken the extraordinary step of overruling the independent determination of the [inspector general] and preventing the disclosure from reaching the committee.
“We do not know whether this decision to withhold the disclosure was made only by you, or whether it involved interference by other parties, including the White House."
Mr Schiff added that the failure to pass on the complaint “raises the prospect that an urgent matter of a serious nature is being purposefully concealed from the committee.”
Dan Coats, the former director of national intelligence, resigned in July after reportedly clashing with the president about matters including US relations with Russia and North Korea.
Mr Maguire has defended his refusal by asserting that the subject of the complaint is beyond his jurisdiction.
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He has agreed to testify before the intelligence committee in a classified session on 26 September.
Mr Atkinson is scheduled to appear at a closed hearing of the committee on Thursday.
The White House has not commented on the complaint.
Additional reporting by agencies