Ships in the area have reported spoofed communications from "unknown entities falsely claiming to be US or coalition warships."
Iranian President Hassan Rouhani warned on Tuesday that shipping might not be safe in the Strait of Hormuz, adding that "war with Iran is the mother of all wars."
Iran has already seized a UK tanker in response and London has said it will attempt to protect its ships from more Iranian harassment.
Earlier on Wednesday, Iran's pragmatist president, Hassan Rouhani said Iran was ready for "just negotiations" but not if they mean surrender.
France, Italy, the Netherlands and Denmark support a European-led naval mission to ensure safe shipping through the Strait of Hormuz.