Donald Trump has confirmed in a series of tweets that he shared information with Sergey Lavrov and Sergey Kislyak, respectively Russia's foreign minister and ambassador in Washington, DC, while hosting them in the Oval Office.
The US president defended his move on Tuesday, saying he had an "absolute right" to do so as commander-in-chief and that he discussed "facts pertaining to terrorism and airline flight safety".
A report by the Washington Post newspaper quoted two US officials saying that Trump shared highly classified information about the Islamic State of Iraq and the Levant (ISIL, also known as ISIS), supplied by a US ally in the fight against the hardline group.
Several major US media outlets identified Israel as the ally that had provided the intelligence to the Trump administration. Israeli officials declined to comment.
Trump's tweets did not say whether he revealed classified information about ISIL.
In a Tuesday press briefing, HR McMaster, Trump's national security adviser, said "the premise of the article was false".
He said sharing the information was "wholly appropriate" and that it did not result "in any way in a lapse in national security".
Al Jazeera's James Bays, reporting from Washington, DC, said "it's clear that he said things to the Russians in the Oval Office", adding the disclosure might "burn a relationship with one of those close allies [in the Middle East]".
As for McMaster's statements, Al Jazeera's Kimberly Halkett, also reporting from Washington, DC, said they are a "very carefully crafted set" and that there is no denial that "classified information was shared".
White House reaction
The disclosure put a source of intelligence on ISIL at risk, according to the Washington Post, which broke the story on Monday.
The White House reacted to the reports, but did not deny that classified information was disclosed in the May 10 meeting between Trump and the Russian diplomats.
Trump entered the debate personally after Rex Tillerson, US secretary of state, and McMaster issued statements saying that no sources, methods or military operations were discussed at the Russian meeting.
The Kremlin dismissed the reports as "complete nonsense". Russia's foreign ministry spokesperson took to Facebook to describe the reports as "yet another fake".
The developments come close on the heels of Trump's contentious dismissal of James Comeyas FBI director amid an investigation into allegations of the president's ties to Russia.
Kislyak, the Russian ambassador, has been a central player in that controversy, involving possible coordination between Trump's campaign and alleged Russian medding in the US presidential election.
'Very, very troubling'
The alleged Trump disclosure comes in advance of his first overseas trip to the Middle East and then to the NATO leaders' summit in Brussels.
Bob Corker, the Republican head of the Senate Foreign Relations Committee, called the allegations "very, very troubling".
"Obviously, they're in a downward spiral right now," he said on Monday, "and they've got to come to grips with all that's happening."
Malcolm Turnbull, Australia's prime minister, would not comment on the disclosure. Australia is a member of the Five Eyes intelligence-sharing programme with the US, Canada, Britain and New Zealand.
Jordan said King Abdullah II was to speak by phone on Tuesday with Trump.
The Royal Court insisted the arrangements for the call were made last week. Jordan is one key US ally in the Middle East.
A senior German legislator expressed concern about the reports.
"If it proves to be true that the American president passed on internal intelligence matters that would be highly worrying," Burkhard Lischka told The Associated Press news agency.