US Defense Secretary Ashton Carter offered assurances Friday that the Iran nuclear deal will leave Washington with a "more effective" military option if that becomes necessary.

Carter defended the agreement in an opinion piece in USA Today, arguing it would lead to a more durable outcome.

But the Pentagon chief said the military option was still on the table and, if called for, such action would be "more effective" as a result of the agreement.

World powers and Iran struck the landmark deal in July after years of negotiations to address Western concerns that Tehran was developing a nuclear bomb.

"Its implementation will block the pathways Iran could take to build a nuclear bomb. That long-term outcome is more durable than one a military strike would bring about," Carter wrote.

"While I am responsible for that alternative and know that it would be effective at setting back Iran's nuclear program, it would do so with potentially serious second- and third-order repercussions, and the likely need to repeat attacks once Iran sought to rebuild its capability."

But Carter argued the deal also made the option of military action by the US against Iran stronger going forward.

"The military option is real today and, as secretary of defense, I will be sure that remains true well into the future," he wrote.

"Indeed, the reality is that any prospective military option, if called for, will be more effective under this deal -- not less. Iran will have a smaller and more concentrated civil nuclear program, and the deal's verification provisions will give us more information with which to plan."

US President Barack Obama earned enough backing in Congress this week to make sure the deal is not blocked by lawmakers. The agreement will ease Western sanctions on Iran in return for Tehran curtailing its nuclear program.

Many Republicans, however, believe the Islamic republic will still seek to cheat its way to developing an atomic bomb.

A public relations battle was launched over the agreement, which is to be put to a vote in Congress next week.

Tehran has steadfastly denied the claims it is developing a nuclear bomb and argued that its nuclear capabilities were for civilian use.