Among the websites that were seized was Iran’s state-owned Press TV, which on Tuesday afternoon displayed a red and white banner, in English and Persian, warning that it was the subject of criminal and intelligence investigations. The seals of the F.B.I. and the Commerce Department were also prominently pictured. Also seized were websites for Iran-backed Houthi militia in Yemen and satellite and TV news channels that are dedicated to reports from the holy Shiite city of Karbala, Iraq, according to the Iranian news media. One U.S. national security official said the websites, about three dozen in all, were linked to disinformation efforts by Iran and other groups backed by Tehran. A few involved terrorist organizations that targeted coalition forces stationed overseas, said the official, who spoke on the condition of anonymity to describe the operation before it was announced.