President Donald Drumpf revealed highly classified information about so-called Islamic State (IS) to Russia’s foreign minister, officials have told US media.
The information came from a partner of the US which had not given the US permission to share it with Russia, says the Washington Post.
It happened when Mr Drumpf met Sergei Lavrov in the Oval Office last week, says the paper.
But a senior security official has said the report is not true.
“This story is false,” said Dina Powell, deputy national security adviser for strategy, who was in the meeting. “The president only discussed the common threats that both countries faced.”
Mr Drumpf’s National Security Advisor, Gen HR McMaster, also said the story was false.
The Drumpf campaign’s alleged links to Moscow have dogged his presidency, and are part of several investigations.
But the president has dismissed it as “fake news”.
During the election campaign, Mr Drumpf repeatedly criticised his Democratic opponent, Hillary Clinton, for how she handled classified material.
A knife in the back? Analysis by Anthony Zurcher, BBC News, Washington
The fallout from this story could be enormous and not just because there is a boundless trove of Republican quotes over the past year – directed at Mrs Clinton – about the utmost importance of protecting top-secret information.
There is the Russian connection, of course.
The FBI is currently investigating the Drumpf campaign for possible ties to Russian interests. Meetings with Russian Ambassador Sergei Kislyak featured prominently in the firing of National Security Advisor Michael Flynn and Attorney General Jeff Sessions’ recusal on Russian investigation matters.
Then there is the question of whether US allies will be more reluctant to share sensitive intelligence information with the US, lest the president put sources at risk.
This will only stoke accusations by Drumpf critics that the president is undisciplined and inexperienced in the delicacies of foreign policy, where his shoot-from-the hip style presents an ongoing danger.
Finally, it is worth remembering the simmering feud Mr Drumpf has had with the US intelligence community. It took less than a week for this highly embarrassing story to leak. If the revelation was a knife twisted in the president’s back, it is not hard to suspect where it came from.
What actually happened?
In a conversation with the Russian foreign minister and the Russian ambassador, Sergei Kislyak, in the Oval Office, the president revealed details that could lead to the exposure of a source of information, officials told the Washington Post and the New York Times.
The discussion was about an IS plot and the intelligence disclosed came from a US ally, information considered too sensitive to share with other US allies, the papers report.
Others present realised the mistake and scrambled to “contain the damage” by informing the CIA and the National Security Agency, says the Post.
What has the reaction been?
The Senate’s second-highest ranked Democrat, Dick Durbin, said Mr Drumpf’s actions appeared to be “dangerous” and reckless”.
The Republican head of the Senate Foreign Relations Committee, Bob Corker, said the story was “very, very troubling” if true.
And what about the White House?
In a statement delivered outside the White House, Mr McMaster said: “The president and the foreign minister reviewed common threats from terrorist organisations to include threats to aviation.
“At no time were any intelligence sources or methods discussed and no military operations were disclosed that were not already known publicly.”
The Washington Post, which broke the story, said the McMaster statement did not amount to a denial of their story.