Donald Trump disclosed highly classified information to the Russian foreign minister during their meeting last week, it has emerged.
The President divulged sensitive information including details of a planned terror operation against Islamic State to Sergey Lavrov.
Trump's alleged actions have potentially jeopardised a source of intelligence about Islamic State, The Washington Post reported .
The newspaper said the information Trump relayed to Russian Foreign Minister Lavrov and Russian Ambassador Sergei Kislyak had been provided by a US partner through a highly sensitive intelligence-sharing arrangement.
The partner had not given Washington permission to share the material with Moscow, and Trump's decision to do so risks cooperation from an ally that has access to the inner workings of the Islamic State militant group, the Post said.
During his Oval Office meeting with Lavrov and Kislyak, Trump went off-script and began describing details about an Islamic State threat related to the use of laptop computers on aircraft, unnamed US officials told the Post.
While discussing classified matters with an adversary would be illegal for most people, the president has broad authority to declassify government secrets, making it unlikely that Trump's disclosures broke the law, the Post said.
Trump's alleged actions were branded "dangerous" and "reckless" by the US Senate's number two Denocrat.
Dick Durbin told reporters: "This conduct by the president is not only dangerous, it's reckless. It is reckless for him to disclose to the Russian foreign minister and ambassador, sensitive, top secret information."
Durbin said he had not been briefed separately on the matter and that he was commenting on the Post story.
He added that classified information cannot be kept from a US president but he hoped Republicans will make it clear to Trump that his conduct "jeopardises our national security."
Secretary of State Rex Tillerson leapt to the President's defence, claiming Trump did not discuss "sources, methods or military operations" in his meeting with Lavrov.
In a statement, Tillerson said: "During President Trump's meeting with Foreign Minister Lavrov, a broad range of subjects were discussed among which were common efforts and threats regarding counter-terrorism.
"During that exchange the nature of specific threats were discussed, but they did not discuss sources, methods or military operations."
Trump's meeting with Lavrov and Kislyak at the White House came a day after he fired FBI Director James Comey, who was leading the agency's investigation into possible links between Trump's presidential campaign and Moscow.
Asked about the disclosures, Trump's national security adviser, H.R. McMaster, who participated in the meeting, said no intelligence sources or methods were discussed that were not already known publicly, the Post reported.
McMaster later addressed reporters to categorically deny the Washington Post's story.
He said: "There is 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 President and the 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 methods discussed.
"And the President did not disclose any military operations that were not already publicly known. Two other senior officials who were present, including the Secretary of State, remember the meeting the same way and have said so.
"The on the record accounts should outweigh anonymous sources. I was in the room, it didn't happen. "
US officials told Reuters that US agencies are in the process of drawing up plans to expand a ban on passengers carrying laptop computers onto US-bound flights from several countries on conflict zones due to new intelligence about how militant groups are refining techniques for installing bombs in laptops.
So serious are assessments of the increased threat that Washington is considering banning passengers from several European countries, including Britain, from carrying laptops in a cabin on US-bound flights.
The United States has consulted about the intelligence with allied governments and airlines.