Iran warned it will take further steps to breach the 2015 nuclear accord in early September if it doesn’t receive long-sought relief from U.S. economic sanctions, as it began enriching uranium above limits set out in the deal.

Foreign Ministry spokesman Abbas Mousavi said on Monday that Iran is prepared to take harder and more steadfast measures than the modest steps it has recently taken to expand its nuclear program, setting in motion what is likely to become months of escalating tension with Washington.

President...