The Republican also said: "We're four years closer to a nuclear Iran", although he softened the uncompromising tone that has been the hallmark of his campaign by emphasising that military action should be a last resort.

BBC: US election debate: Candidates spar on foreign policy

Alarmed by months of sabre-rattling and a possibility that Israel will make good on long-standing hints that it intends to attack Iran's nuclear facilities, he and President Barack Obama had softened their tone before the talks begin.

ECONOMIST: Iran