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.