US Secretary of State John Kerry was due to arrive in Paris Saturday seeking to boost European support for an emerging nuclear deal with Iran as France voiced concerns over whether it was stringent enough.
Kerry was to fly in from London on the last stop of his latest diplomatic trip, to brief his French, German and British counterparts on the negotiations.
The top US diplomat spent three days in Switzerland earlier this week huddled with Iranian Foreign Minister Mohammad Javad Zarif 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, remained 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 on the sidelines of an EU foreign ministers meeting in Latvia.
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.
Kerry has acknowledged that he is uncertain whether a deal can be reached, warning that after months of intensive negotiations between world powers and Iran time is running out.
"We are seeking to show that Iran's program is exclusively peaceful and that we can block all of the pathways necessary to acquire the fissile material for a nuclear weapon and then to be able to move towards the production of that 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."
Bilateral negotiations between the US and Iran will resume on March 15.
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.