WASHINGTON - President Trump boasted about highly classified intelligence in a meeting with the Russian foreign minister and ambassador last week, providing details that could expose the source of the information and the manner in which it was collected, a current and a former American government official said Monday.
The intelligence disclosed by Mr. Trump in a meeting with Sergey V. Lavrov, the Russian foreign minister, and Sergey I. Kislyak, the Russian ambassador to the United States, was about an Islamic State plot, according to the officials. A foreign ally that closely guards its own secrets provided the information, which was considered so sensitive that American officials did not share it widely within the United States government or pass it on to other allies.
Mr. Trump's disclosure does not appear to have been illegal - the president has the power to declassify almost anything. But sharing the information without the express permission of the ally who provided it represented a major breach of espionage etiquette, and could jeopardize a crucial intelligence-sharing relationship.
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The ally has repeatedly warned American officials that it would cut off access to such sensitive information if it were shared too widely, said the former official. In this case, the fear is that Russia will be able to determine exactly how the information was collected and could disrupt the ally's espionage efforts.
The Washington Post first reported the disclosure.
Beyond angering a partner and calling into question the ability of the United States to keep secrets, the episode also opens Mr. Trump to criticism of a double standard. The president made Hillary Clinton's mishandling of classified information through her private email server central to his campaign, leading chants of "lock her up" at rallies. But there was never any indication that Mrs. Clinton exposed sensitive information from an ally or gave it to an adversary.
The Trump administration pushed back on the revelation, with high-ranking officials denying that the president discussed such highly sensitive information with the Russian officials.
"The president and the foreign minister reviewed common threats from terrorist organizations to include threats to aviation," said H. R. McMaster, the national security adviser, who attended the meeting. "At no time were any intelligence sources or methods discussed, and no military operations were disclosed that were not already known publicly."
Secretary of State Rex W. Tillerson echoed General McMaster, saying a range of subjects came up, including "the nature of specific threats," but not sources, methods or military operations.
But according to the officials, Mr. Trump discussed the contents of the intelligence, not the sources and methods used to collect it. The concern is that knowledge of the information about the Islamic State plot could allow the Russians to figure out the sources and methods.
"This story is false. The president only discussed the common threats that both countries faced," said Dina Powell, the deputy national security adviser, who was also at the meeting.
The episode immediately reverberated around Washington.
"To compromise a source is something that you just don't do, and that's why we keep the information that we get from intelligence sources so close as to prevent that from happening," Senator Bob Corker, Republican of Tennessee and the chairman of the Foreign Relations Committee, told reporters.
Senator Mark Warner, Democrat of Virginia and the vice chairman of the 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."
The Central Intelligence Agency declined to comment.