Britain and France insisted Monday that any deal on Iran's contested nuclear programme must include a comprehensive verification regime to ensure Tehran sticks to its commitments, as the clock ticks down to an end-of-month deadline.

After years of tortuous negotiations, both sides are racing to agree a deal by June 30 that would see Iran open up its nuclear programme to allay concerns it is seeking atomic weapons, in return for the West lifting punishing economic sanctions.

But suspicions run deep and both British foreign secretary Philip Hammond and his French counterpart Laurent Fabius were adamant that Iran, which says its nuclear programme is only for peaceful purposes, should have no wriggle room.

"Britain wants a robust and verifiable deal with Iran that ensures that its nuclear programme in the future is exclusively civil," Hammond said after talks in Luxembourg with his Iranian counterpart Mohammad Javad Zarif, Fabius, Germany's Frank-Walter Steinmeier and EU foreign affairs head Federica Mogherini.

"We cannot compromise on the absolute red lines we have. If we do a deal, it has to be verifiable," Hammond said.

"There is a lack of trust on both sides and only full verification... to confirm that both sides are in compliance with their agreements is going to regenerate that trust in the future."

Fabius had already warned Sunday that any deal with Iran had to be verifiable 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.

Just before the talks, Zarif had said that "all sides should avoid excessive demands ... so as to allow us to reach an accord" and afterwards he appeared confident of progress.

"There is a political commitment on the part of everybody to move forward," Zarif said on the sidelines of an EU foreign ministers meeting in Luxembourg.

"There is the possibility that we can finish this by the deadline or a few days after the deadline," he said.

Mogherini, who as European Union foreign affairs head has been brokering the negotiations, said she had "a useful meeting with Zarif."

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 if regular inspections confirm that Tehran is sticking to the accord.

The two sides, having missed a March 31 deadline, agreed in early April a new date of June 30 to finalise the accord and negotiators have been meeting regularly in Vienna and elsewhere since then to hammer out a deal.

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April