US Central Command chief General Joseph Votel told a Senate panel he shared the views of Defense Secretary Jim Mattis and Joe Dunford, the chairman of the Joint Chiefs of Staff.

"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.

He cited disagreements on the issue as a reason for his decision to fire on Tuesday his diplomatic chief Rex Tillerson and replace him with CIA Director Mike Pompeo, who is considered hawkish.

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.

Struck in 2015, it was signed by Iran with the five permanent members of the United Nations Security Council -- Britain, China, France, Russia and the United States -- plus Germany.

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.