WASHINGTON — Secretary of State Rex W. Tillerson described a landmark Iran nuclear deal as a failure on Wednesday, only hours after the State Department said Tehran was complying with its terms. But the top United States diplomat stopped short of threatening to jettison the 2015 agreement that was brokered by world powers, or saying whether the Trump administration would punish Iran with new sanctions.

The whiplash left Republicans on Capitol Hill, who had universally excoriated the agreement to limit Iran’s nuclear program and voted against its implementation, uncertain of how to respond. Its architects, however, said they were cautiously optimistic that the deal would stay in place.

The nuclear deal “fails to achieve the objective of a non-nuclear Iran,” Mr. Tillerson said. “It only delays their goal of becoming a nuclear state.”

He said that Iran continued to threaten the United States and the rest of the world, and he announced that the Trump administration was reviewing ways to counter challenges posed by Tehran.