President Donald Trump revealed highly classified information about so-called Islamic State (IS) to Russia’s foreign minister, sources have told US media.
The information 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.
Officials denied the media report but a senior member of Mr Trump’s Republican party called for an explanation.
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.
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.
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.
The Senate’s second-highest ranked Democrat, Dick Durbin, said Mr Trump’s actions appeared to be “dangerous” and reckless”.
“The president and foreign minister reviewed a range of common threats to our two countries, including threats to civil aviation,” he said.