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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 Advisor 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.
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.
What has the White House said?
National Security Adviser HR McMaster told reporters that the story, "as reported", was "false".
"The president and foreign minister reviewed a range of common threats to our two countries, including threats to civil aviation," he said.
"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."
In a statement, Secretary of State Rex Tillerson echoed the point that "the nature of specific threats were discussed, but they did not discuss sources, methods or military operations".
The Washington Post, which first broke the story, said this did not amount to a denial.
Speaking to the BBC, reporter Greg Jaffe said the Post's story made it clear the president did not disclose sources or methods.
But he added: "Our story says that the nature of the information provided would have allowed the Russians to 'reverse engineer' to discover the sources and methods. He said so much that they could figure it out."
What has the reaction been?
The Senate's second-highest ranked Democrat, Dick Durbin, said Mr Trump's actions appeared to be "dangerous" and reckless".
A spokesman for Paul Ryan, Republican speaker of the House of Representatives, said: "We have no way to know what was said, but protecting our nation's secrets is paramount.
"The speaker hopes for a full explanation of the facts from the administration."
The Republican head of the Senate Foreign Relations Committee, Bob Corker, said the story was "very, very troubling" if true.
"Obviously they're in a downward spiral right now and they've got to figure out a way to come to grips" with it, he told Bloomberg.