"There will need to be some more flexibility shown by our Iranian partners if we are going to reach a deal," British foreign secretary Philip Hammond said.
"I think the serious negotiations are now getting underway and over the next week, I hope to start to see some progress," Hammond said.
"We are pushing hard to get there now."
Hammond was speaking as he arrived for a meeting of European Union foreign ministers in Luxembourg where he will also hold talks with Iranian Foreign Minister Mohamed Javad Zarif.
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Fabius warned yesterday that any deal with Iran had to be tough and verifiable to ensure no backsliding.
"We think that we must be extremely firm and that, if an agreement is to be reached, that agreement must be robust," Fabius said at a joint news conference with Israeli Prime Minister Benjamin Netanyahu in Jersusalem.
"That means that it must be able to be verified," Fabius added.
Iran and the P5+1 powers -- Britain, China, France, Germany, Russia and the United States -- agreed in April on the main outlines of what would be a historic deal scaling down Tehran's nuclear programme.
The west fears that Iran's nuclear programme has military objectives but Tehran insist is for purely peaceful energy development purposes.