"From my perspective, the JCPOA addresses one of the principal threats that we deal with from Iran," Votel said, using the deal's official acronym.
"So, if the JCPOA goes away, then we will have to have another way to deal with the nuclear weapons program."
Trump is threatening to scrap the international agreement unless tough new restrictions were placed on Iran before May 12.
The president is concerned that parts of the deal start to expire from 2026 and that it fails to address Iran's missile program, its regional activities or its human rights abuses.
A US exit could kill the nuclear pact, which the Islamic republic has refused to re-negotiate.
Under the agreement, Iran agreed to freeze its nuclear program in return for the lifting of punishing international sanctions.
While Iran has reaped massive economic benefits from the accord, notably by being able to resume oil exports, it is still constrained by US sanctions in other areas.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)