He flew in from Geneva where three days of marathon talks which stretched into early today failed to produce an interim deal under which Iran would suspend parts of its enrichment programme in return for sanctions relief.
Despite leaving without an agreement after his surprise decision to re-route his 10-day Middle East tour via the Swiss city, the top US diplomat insisted western powers were "closer now" to ending the decade-long standoff with Tehran.
"A number of nations -- not just the French, but ourselves and others -- wanted to make sure that we had the tough language necessary" to ensure the six powers did not do "something sloppily that could wind up with a mistake," he told NBC's "Meet the Press".
Reiterating the P5+1 group, comprising Britain, China, France, Germany, Russia and the United States, was "not in a rush," Kerry said the aim was "to lock the programme in where it is today -- in fact, set it back -- while one negotiates the full deal."
Kerry was due to hold dinner talks with Abu Dhabi Crown Prince Mohammed bin Zayed Al-Nahayan and Foreign Minister Abdullah bin Zayed Al-Nahayan, before heading home to Washington tomorrow at the end of an exhausting seven-nation tour.
The UAE like its Saudi neighbours has reacted warily to the cautious US rapprochement with the new Iranian regime of President Hassan Rouhani.
But Kerry insisted Iran would be under "tremendous pressure" to comply with a deal as "the core architecture" of oil and banking sanctions would remain.
