KYIV, April 10 (Reuters) - Ukraine has been forced to amend some of its military plans ahead of a much-vaunted counter-offensive due to a leak of classified U.S. documents, CNN reported on Monday citing a source close to Ukrainian President Volodymyr Zelenskiy.

Ukrainian officials told Reuters on Friday the allegedly leaked documents contained fictitious information and looked like a Russian disinformation campaign.

U.S. officials are trying to find the source of a leak of classified documents detailing a wide array of topics from Ukraine's air defences to Israel's Mossad spy agency.

Reuters has not independently verified the documents' authenticity.

Asked about the CNN report, presidential aide Mykhailo Podolyak said Ukraine's strategic plans remained unchanged but that more specific tactical plans were always subject to change.

"There are strategic tasks - they are unchangeable," he told Reuters.

"But operational and tactical scenarios are constantly refined, based on an assessment of the situation on the battlefield, resource provision, intelligence data on the enemy's resources, etc." he said.

"Right now its impossible to reassess plans, because they are only being worked out (now)," he added.

Oleksiy Danilov, secretary of the National Security and Defense Council, told Reuters: "We are working on our own plans... The opinion of people who have nothing to do with this do not interest us...The circle of people who possess information is extremely restricted."

Some experts have said the source of the leak could be American.

"The focus now is on this being a U.S. leak, as many of the documents were only in U.S. hands," Michael Mulroy, a former senior Pentagon official, told Reuters in an interview.

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