Top US diplomat John Kerry arrived in Paris Saturday to boost European support for a deal with Iran as France voiced concerns over whether it would prove tough enough to stop Tehran from acquiring a nuclear weapon.
Flying in from London on the last stop of a week-long trip, Kerry was to brief his French, German and British counterparts on the negotiations.
The US secretary of state 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 a March 31 deadline looms.
But French Foreign Minister Laurent Fabius, who will meet first with Kerry for bilateral talks on Saturday, remains sceptical.
"There has been progress but as regards the numbers, controls and the length of the agreement, the situation is still not sufficient, so there is more work to be done," Fabius said Friday.
Fabius gave no figures 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.
"The deadline is March 31 but in the event it could be later, although everyone wants to make progress by the end of the month," Fabius said.
Fears have been raised both in Europe and Israel that the deal could leave much of Iran's uranium enrichment capability in place -- in particular that Tehran would still have thousands of centrifuges which spin uranium gas into high-grade fissile material capable of fuelling a nuclear weapon.
Iran has long denied seeking to arm itself with an atomic bomb, insisting its nuclear programme is for peaceful civilian purposes.
Kerry has acknowledged that he is uncertain whether a deal can be reached, warning that time is running out amid domestic pressures both in the US and Iran.
"We are seeking to show that Iran's programme is exclusively peaceful and that we can block all of the pathways necessary to acquire the fissile material for a nuclear weapon," Kerry said on Thursday after briefing Gulf foreign ministers in Riyadh on the state of the talks.
"To date, we have made progress, but there do remain serious gaps, and those need to be resolved," he stressed.
"It may be that Iran cannot say yes to the type of deal that provides assurances that the international community requires."
But 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.
"We expect soon thereafter to know whether Iran will, in fact, be able to make the tough decisions that are required to get where we need to be," Kerry said.