Negotiators of Iran and six world powers face each other at a table in the historic basement of Palais Coburg hotel in Vienna April 24, 2015. (Reuters Photo)
Britain and France have pressed Iran hard to agree a comprehensive deal on its contested nuclear programme as an end-June deadline looms after years of tortuous negotiations with the West.
"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, also attended by Iranian Foreign Minister Mohamed Javad Zarif.
In comments carried on the website of Iran's state television, Zarif said "All sides should avoid excessive demands ... so as to allow us to reach an accord."
"There are still differences, some of them technical and others political, but we are trying to finish the work as early as possible," he was quoted as saying on his arrival in Luxembourg.
Zarif was due to hold talks with EU counterparts Hammond plus France's Laurent Fabius, Germany's Frank-Walter Steinmeier and EU foreign affairs head Federica Mogherini.
Fabius warned on Sunday that any deal with Iran had to be robust and verifiable to ensure no backsliding and on Monday spelt out France's demands again.
"A robust agreement is one which includes an extensive verification element, including if necessary visits to military sites and automatic re-introduction of sanctions if Iran violates the accord," Fabius told reporters.
Germany's Steinmeier said separately that the talks were now at a decisive moment as both sides drafted a final document.
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.
In return, they agreed that punishing western sanctions against Iran would be progressively lifted as inspectors confirmed that Tehran was sticking to the accord.
The two sides, having missed a March 31 deadline, set a new date of June 30 to finalize the accord and negotiators have been meeting regularly in Vienna and elsewhere since then to hammer out a deal.
The West fears that Iran's nuclear programme has military objectives but Tehran insists it is for purely peaceful purposes.