Carter met with the king as well as his defense minister, Mohammad bin Salman Al Saud, Wednesday in the Red Sea city of Jedda.
Carter told reporters afterward that the king expressed reservations about how well Iranian compliance with the nuclear deal can be verified. And he expressed concern about the mechanism for reinstating international economic sanctions in the event that Iran is found to have violated the deal.
"Those are the same issues that we know will arise" during implementation of the agreement, Carter said. But he also said that both men reiterated their support for the nuclear deal.
The agreement was struck last week among Iran and six major world powers.
Under the deal, Iran will curb its nuclear program in exchange for the easing of economic penalties levied by the United States, the United Nations and the European Union over the next decade. If Iran does not comply with the terms of the deal or allow verification that it is complying, the Obama administration says the sanctions can be reinstated or "snapped back." Experts say reinstating sanctions will be more difficult than that.
Saudi Arabia sees Iran as its chief regional foe and is worried about growing Iranian influence in Yemen and elsewhere in the Persian Gulf region.
Carter also said King Salman will visit the United States this fall. It would be his first trip to Washington since assuming the throne in January.