President Donald Trump revealed highly classified information about so-called Islamic State (IS) to Russia’s foreign minister, US media report.
The information, related to the use of laptops on aircraft, came from a partner of the US which had not given permission for it to be shared with Russia, says the Washington Post.
Mr Trump received Sergei Lavrov in the Oval Office last week.
National Security Adviser HR McMaster dismissed the reporting as “false”.
The Trump campaign’s alleged links to Moscow have dogged his presidency and are part of several investigations.
But the president has dismissed such allegations as “fake news”.
During the election campaign, Mr Trump repeatedly criticised his Democratic opponent, Hillary Clinton, for how she handled sensitive 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 Trump campaign for possible ties to Russian interests. Meetings with Russian Ambassador Sergei Kislyak featured prominently in the firing of National Security Adviser Michael Flynn and the recusal of Attorney General Jeff Sessions 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 Trump 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 Trump 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 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. The president reportedly went “off-script”, revealing specifics of the plot, thought to centre on the use of laptop computers on aircraft, and the city from which that threat had been detected.