An initial U.S. assessment indicated Iran likely was behind the attack on two Saudi Arabian oil tankers and two other vessels damaged over the weekend near the Strait of Hormuz, a U.S. official said, a finding that, if confirmed, would further inflame military tensions in the Persian Gulf.

The assessment, while not conclusive, was the first suggestion by any nation that Iran was responsible for the attack and comes after a series of U.S. warnings against aggression by Iran or its allies and proxies against military or commercial...