Iran has yet to prove to the world it wants a peaceful nuclear programme, top US diplomat John Kerry said on Saturday, warning with his French counterpart that gaps still had to be overcome to clinch a deal.
"Critical weeks" lie ahead as a March 31 deadline looms to seal a political framework on reining in Iran's suspect atomic programme, Kerry said after meeting with French foreign minister Laurent Fabius.
"We want an agreement that's solid," Kerry told reporters ahead of four-way talks to also include German Foreign Minister Frank-Walter Steinmeier and their British counterpart Philip Hammond.
"We want an agreement that will guarantee that we are holding any kind of programme that continues in Iran accountable to the highest standards so that we know in fact that it is a peaceful programme."
The US secretary of state flew in from London on the last stop of a week-long trip to brief his European counterparts on the state of the negotiations.
France has privately expressed concern that Washington could be rushing into a deal without ensuring iron-clad guarantees are in place to stop Tehran acquiring a nuclear bomb.
But in a show of unity, Kerry said he had "the same assessment" as Fabius. "We have made progress, but there remain gaps, divergences as he (Fabius) said, and we need to close those gaps," Kerry insisted. "And that is our goal over the course of the next days. We have a critical couple of weeks ahead of us."
Fabius also highlighted that progress had been made in months of talks since an interim deal reached on November 2013, but stressed "differences still remain" which had to be "overcome" and "there is still work to do."
Kerry spent three days in Switzerland earlier this week huddled with Iranian foreign minister Mohammad Javad Zarif in the latest round of intense negotiations seeking to hammer out a deal as the deadline looms.
'Up to Iran'
Fabius on Friday had appeared sceptical about the state of the negotiations saying "as regards the numbers, controls and the length of the agreement, the situation is still not sufficient."
He refused to go into specifics on Saturday.
But key issues in the talks which began in late 2013 include the level of uranium enrichment that Iran should be allowed, the degree of international oversight of its programme and how long an accord should last.
"It is frankly up to Iran that wants this programme... that asserts that they have a peaceful programme to show the world that it is indeed exactly what they say," said Kerry.
Iran has long denied seeking to arm itself with an atomic bomb, insisting its nuclear programme is for peaceful civilian purposes.
Iran's nuclear chief Ali Akbar Salehi, who met this week with the US Energy Secretary Ernest Moniz, insisted "we have overcome the stalemate over technical issues."
The unfinished plutonium reactor at Arak as well as the uranium enrichment site of Fordo had both figured high in these week's discussions, he said.
"On enrichment and Arak, we have made very good progress. We have replied to their concerns ... by making technical proposals while also defending our national interests and our nuclear industry," Salehi told Iran television, without giving specifics.
US-Iran bilateral talks are due to resume on March 15, most likely in Geneva.
Further talks between the group known as the P5+1 -- Britain, China, France, Germany, Russia and the United States -- and Tehran are also expected.