CNN is confirming Monday based on one of the documents which appeared online that the US has been spying on Ukrainian President Vladimir Zelensky - a disclosure which has caused officials in Kiev to be "deeply frustrated".

"One document reveals that the US has been spying on Zelensky," CNN reports. "That is unsurprising, said the source close to Zelensky, but Ukrainian officials are deeply frustrated about the leak."

The US intelligence document suggests that American officials have been worried about possible Zelensky decision-making to strike deep into Russian territory, which would escalate the war and potentially bring Russian and NATO into direct clashes:

The US intelligence report, which is sourced to signals intelligence, says that Zelensky in late February "suggested striking Russian deployment locations in Russia's Rostov Oblast" using unmanned aerial vehicles, since Ukraine does not have long-range weapons capable of reaching that far.