President Donald Trump 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 Trump met Sergei Lavrov in the Oval Office last week, says the paper.
But a senior security official has said the report is not true.
"The story that came out tonight, as reported, is false," National Security Advisor HR McMaster told reporters outside the White House.
"The president and foreign minister reviewed a range of common threats to our two countries, including threats to civil aviation.
"At no time, at no time, were intelligence sources or methods discussed, and the president did not disclose any military operations that were not already publicly known."
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, Trump repeatedly criticised his Democratic opponent, Hillary Clinton, for how she handled sensitive material.