"We came to Geneva determined to make certain that Iran does not acquire a nuclear weapon. That remains our goal," Kerry told reporters after three days of intense diplomatic efforts failed to produce a breakthrough in the decade-long dispute.
While no deal was reached, Kerry insisted negotiators had "made significant progress," in Geneva.
"There's no question in my mind that we are closer now (to a deal) as we leave Geneva," he said.
His comments came after EU diplomatic chief Catherine Ashton announced that Iran and six world powers had not been able to finalise a deal and that negotiations would resume on .
Kerry, who had cut short a Middle East tour to throw his weight behind the talks, insisted he was not discouraged to be leaving without an agreement.
"It takes time to build confidence between countries who have been really at odds for a long time," he explained, pointing to the distrust that has reigned between Washington and Tehran since the 1979 Iranian revolution."
Following a decade-long standoff over Iran's nuclear programme, Kerry insisted the marathon Geneva talks "narrowed the differences and clarified those that remained".
The P5+1 group of world powers, comprising permanent UN Security Council members — Britain, China, France, Russia and the United States — plus Germany, suspect Iran's programme is aimed at developing nuclear weapons, despite Tehran's repeated denials.
"We are working hard to try to overcome mistrust, to try to build confidence," Kerry said.
Negotiators were trying to find a way "to achieve this goal of ascertaining with certainty, without doubt, that the programme is a peaceful nuclear programme."