President Barack Obama has pulled off a historic deal with Iran on curbing its nuclear programme but he and other global leaders now have a difficult job ahead turning an interim accord into a comprehensive agreement.
In a sign of how difficult the coming talks will be, some differences emerged between US Secretary of State, John Kerry, and his Iranian counterpart, Mohammad Javad Zarif, in their public presentation of a key part of the deal - whether or not Iran preserved the right to enrich uranium.
Obama also has to persuade its ally Israel, whose Prime Minister Benjamin Netanyahu denounced the deal as a "historic mistake," that the accord will reduce and not increase the threat from its arch foe Iran. And he has to sell the accord to skeptics in Congress, including some in his own Democratic Party, who have been pressing for more sanctions on Iran.
Speaking on Iran's state-owned Press TV on Sunday, Zarif said: "In the final step, the (uranium) enrichment process will be accepted and at the same time all the sanctions will be lifted."
However, on the ABC News programme "This Week," Kerry stressed that such a right would be limited and would come about as a result of future negotiations.
He said that under the terms of the agreement, "there will be a negotiation over whether or not they could have a very limited, completely verifiable, extraordinarily constrained programme, where they might have some medical research or other things they can do, but there is no inherent right to enrich..."
Arriving in London to meet British foreign minister, William Hague, Kerry acknowledged a tough road lay ahead before a final agreement could be reached.
"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.
Selling the deal
Obama telephoned Netanyahu to reassure him that Washington would continue to stand by Israel and to suggest that the United States and Israel should quickly start consultations.
Obama also has to sell the deal to skeptics in Congress, including some in his own Democratic Party, who have been pressing for more sanctions on Iran.
"A fairer agreement would have coupled a reduction in sanctions with a proportionate reduction in Iranian nuclear capability," said Senator Charles Schumer.
Senators have for months been discussing imposing even tighter sanctions on Iran, which could anger Tehran and put the interim deal reached in Geneva in jeopardy. Pro-Israel lobbying organisations - among the most effective interest groups in Washington - have failed so far to persuade lawmakers to tighten the sanctions screw on Iran.
The agreement does not need to be ratified by Congress and Obama is using his executive power to temporarily suspend some existing US sanctions on Iran.
Zarif flew home from Geneva to a jubilant crowd, a reflection of the relief felt by many Iranians exhausted by isolation and sanctions that have been particularly punishing in the last two years.