Highly classified Pentagon documents leaked online in recent weeks have provided a rare window into how the US spies on allies and foes alike, deeply rattling US officials, who fear the revelations could jeopardize sensitive sources and compromise important foreign relationships.

Some of the documents, which US officials say are authentic, expose the extent of US eavesdropping on key allies, including South Korea, Israel, and Ukraine.

Others reveal the degree to which the US has penetrated the Russian Ministry of Defense and the Russian mercenary organization Wagner Group, largely through intercepted communications and human sources, which could now be cut off or put in danger.

Still, others divulge key weaknesses in Ukrainian weaponry, air defense, and battalion sizes and readiness at a critical point in the war, as Ukrainian forces gear up to launch a counteroffensive against the Russians – and just as the US and Ukraine have begun to develop a more mutually trusting relationship over intelligence-sharing.

There are claims that members of the British armed forces have acted in Ukraine that have been published in a section of the press, citing the leaked classified documents. Yesterday the British Ministry of Defence warned that the leaked information was “inaccurate” and may be part of an effort to spread “disinformation”.

However, media outlets, including the public broadcaster BBC and The Guardian newspaper, report that some of the documents, which appear to be authentic, indicate that up to 50 members of the special forces of the British Army were “deployed” in February and March this year in Ukraine, along with dozens of their colleagues from other Western states. They were almost twice as many as US (14) and French (15) special forces personnel.

The documents, the Guardian points out, do not make clear where exactly the members of the special forces were deployed, nor what exactly they did. Before the war broke out in February 2022, Britain contributed to the training of Ukrainian personnel.

Ukraine has already altered some of its military plans because of the leak, a source close to Ukrainian President Volodymyr Zelensky told CNN.

The Pentagon has stood up an “interagency effort” to assess the impact of the leak, Pentagon deputy press secretary Sabrina Singh said Sunday.

“The Department of Defense continues to review and assess the validity of the photographed documents that are circulating on social media sites and that appear to contain sensitive and highly classified material,” Singh said in a statement. “An interagency effort has been stood up, focused on assessing the impact these photographed documents could have on U.S. national security and on our Allies and partners.”

Singh added that US officials spoke with allies and partners over the weekend regarding the leak, and informed “relevant congressional committees.”

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