The BBC said the documents, almost 50 pages in all, were found "in a soggy heap behind a bus stop in Kent early on Tuesday morning" by a member of the public, who wanted to remain anonymous.
Russia's relations with Washington, which cancelled the deployment of two of its own warships to the Black Sea last week after fierce Russian protests, are at a post-Cold war low.
Moscow expelled 10 US diplomats on Friday in retaliation for the expulsion of the same number of Russian diplomats from the United States over alleged malign activity.