The reports are being denied by the White House, but The Washington Post broke the story today that in a meeting the president told the Russians highly classified security information about ISIS (something about airline safety and laptops).
Among the questions being raised is how this will effect the relationship with the unnamed U.S. partner (later reported by the New York Times to be Israel, although this was not confirmed) from which the U.S. received the information.
The Washington Posts sources for the story are "current and former U.S. officials." They withheld most of the details of the information, but to quote their original piece:
"In his meeting with Lavrov, Trump seemed to be boasting about his inside knowledge of the looming threat. 'I get great intel. I have people brief me on great intel every day,' the president said, according to an official with knowledge of the exchange.
"Trump went on to discuss aspects of the threat that the United States learned only through the espionage capabilities of a key partner. He did not reveal the specific intelligence-gathering method, but he described how the Islamic State was pursuing elements of a specific plot and how much harm such an attack could cause under varying circumstances. Most alarmingly, officials said, Trump revealed the city in the Islamic State’s territory where the U.S. intelligence partner detected the threat."
The White House struck details of the meeting from the official transcripts for a "sort of memo summary" to be used in-house, and controlled dissemination of the transcripts for the wider public as well as immediately contacting the CIA and NSA.