"Now the really hard part begins, and that is the effort to get the comprehensive agreement, which will require enormous steps in terms of verification, transparency, and accountability," he said at a joint press conference with British Foreign Secretary William Hague in London.
Iran agreed to curb its nuclear programme for the next six months in exchange for limited sanctions relief following marathon talks with the five permanent members of the UN Security Council plus Germany in Geneva that ended today.
Both Kerry and Hague attended the Geneva talks before heading to London for their bilateral meeting.
Kerry vowed "to work together" with America's allies, saying "we'll start today, literally, to continue the efforts out of Geneva and to press forward."
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He also said that US President Barack Obama "could not be more grateful" for the support of British Prime Minister David Cameron.
Cameron later said in a statement that the deal moved Iran "further away from getting a nuclear weapon" and called it "an important first step".
"Today's deal with Iran demonstrates how persistent diplomacy and tough sanctions can together help us to advance our national interest," he added.
Kerry spoke ahead of separate talks on Libya and Syria with Hague.
The British minister praised the agreement as "a good deal for the Middle East and for the world".