WASHINGTON—The U.S. said it would send military forces and hardware to Gulf allies and moved to sever some of Iran’s last ties to world markets on Friday, while preparing to outline a case for international action next week when world leaders gather at the United Nations.

The sanctions on Iran’s central bank and two other major state financial institutions were punishment for attacks on critical oil supplies in Saudi Arabia. They came as the administration sought to steer clear of a military response, instead bolstering efforts...