US President Donald Trump revealed highly classified information to the Russian foreign minister and ambassador when they met him at the White House last week.
Current and former US officials said Trump's disclosures jeopardised a critical source of intelligence on the Islamic State.
Officials said that the intelligence was so sensitive that details have been withheld from allies and tightly restricted even within the US government.
The information Trump relayed had been provided by a US partner through an intelligence‑sharing arrangement.
The partner had not given the United States permission to share the material with Russia, and officials said that Trump's decision to do so risks cooperation from an ally that has access to the inner workings of the Islamic State.
After Trump's meeting, senior White House officials took steps to contain the damage, placing calls to the CIA and National Security Agency.
Trump went off script, describing details about an Islamic State terrorist threat related to the use of laptop computers on aircraft.
As US president, Trump has broad authority to declassify government secrets, making it unlikely that his disclosures broke the law.
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