Trump 'shared classified intelligence' with Russia
The Washington Post said Trump had given Russian Foreign Minister Sergei Lavrov and Moscow's envoy to the US Sergey Kislyak details about a specific terror threat posed by the Islamic State group during Oval Office talks last week.
The bombshell report comes as Moscow's alleged interference in last year's US presidential election is back in the spotlight following Trump's shock firing of FBI chief James Comey, whose agency was investigating Russia's possible collusion with aides to the Republican billionaire.
"The president and the foreign minister reviewed a range of common threats to our two countries, including threats to civil aviation," National Security Adviser HR McMaster, who participated in the meeting, told reporters.
"At no time were intelligence sources or methods discussed. And the president did not discuss any military operations that were not already publicly known," he added.
"There's nothing that the president takes more seriously than the security of the American people. The story that came out tonight as reported is false."
The information disclosed by the US leader had been provided by a partner of the United States that had not given Washington authorisation to share it with Moscow, the Post said.
Trump "revealed more information to the Russian ambassador than we have shared with our own allies," the Post quoted a US official as saying on condition of anonymity.
According to the official, the information Trump shared with the Russian officials carried one of the highest levels of classification used by US intelligence agencies.
The Post reported, citing unnamed officials, that "Trump went off script and began describing details about an Islamic State terrorist threat related to the use of laptop computers on aircraft."
The paper said it was withholding details of the plot at the urging of US officials who are concerned it could jeopardise key intelligence capabilities.
The US Department of Homeland Security has said it is mulling the expansion of a ban on laptop computers in passenger cabins on jets originating in Europe.
A US ban is already in place for several countries in the Middle East and North Africa.
Ryan wants 'full explanation'
Trump's closed-door meeting with the Russian officials came a day after his surprise firing of Comey.
The information at issue was not even widely shared within the US government, and Trump's decision to share it with Moscow could endanger Washington's partnership with the source.
Trump reportedly even revealed the city in which the intelligence was gathered - something seen as problematic, as it could allow Moscow to identify the partner nation.
Officials cited by the Post said the partner country had "access to the inner workings of the Islamic State."
Reaction on Capitol Hill was swift.
"We have no way to know what was said, but protecting our nation's secrets is paramount," said Doug Andres, a spokesman for Republican House Speaker Paul Ryan. "The speaker hopes for a full explanation of the facts from the administration."
Senior Republican Senator John McCain told CNN that "if it's true, it's obviously disturbing." But he cautioned: "Let's wait and see what this was all about first."
Mark Warner, the ranking Democrat on the Senate Intelligence Committee, said on Twitter: "If true, this is a slap in the face to the intel community. Risking sources & methods is inexcusable, particularly with the Russians."