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.
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.
The intelligence disclosed came from a US ally and was 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 (NSA), says the Post.
Mr Trump’s actions would not be illegal, as the US president has the authority to declassify information.
The meeting came a day after Mr Trump fired his FBI chief, James Comey, sparking criticism that he had done so because the FBI was investigating alleged Russian ties.