"There is still lots to work out, it's still very fragile," Le Drian told journalists in Paris about the talks between Tehran and three European countries -- France, Britain and Germany -- to keep the nuclear deal alive after US President Donald Trump pulled out of it last year.

Trump later reimposed harsh sanctions that have pummelled the Iranian economy, prompting a defiant Tehran to ramp up its uranium enrichment to levels in breach of the 2015 deal.

France has been leading European efforts to foster dialogue, with President Emmanuel Macron expressing hope during G7 talks in late August of organising a meeting between Rouhani and US President Donald Trump.

However, in a speech to Iran's parliament on Tuesday, President Hassan Rouhani ruled out bilateral talks with the US and threatened a third step away from the deal "in the coming days."

Reacting to the threat, a French diplomat involved in the talks with Tehran said any further violation of the deal would send a "bad signal".

The diplomat, who spoke on condition of anonymity, said such a move by Iran would "make the (mediation) work more complicated" but added that France would pursue its efforts "because there is no other way of getting out of the current escalataion (between the US and Iran) than what we are proposing."

On Monday, an Iranian delegation travelled to Paris for talks with French officials.

Le Drian said the discussion focused on a possible credit line for Tehran in exchange for oil, conditioned on Iran's renewed compliance with the 2015 deal.

Tehran would also have to commit to easing geopolitical tensions in the Gulf region, and participate in Middle East talks on improving regional security, Le Drian said.

"That all supposes of course that President Trump allows waivers on some points" of the new US sanctions on Iran, he added.

A conservative Iranian lawmaker was quoted by media reports at the weekend as saying that Macron had proposed a $15-billion (13.7-billion-euro) credit line, a figure that French officials have not confirmed.

- 'It should work' -

As part of their efforts to keep the nuclear deal alive, France, Germany and Britain have set up a mechanism called INSTEX that would allow continued trade with Iran despite the US sanctions.

"We have to finish the work on INSTEX... normally it should work," Le Drian said.

Macron and Rouhani have held a series of phone calls in recent weeks aimed at salvaging the nuclear deal.

Trump caused surprise at a G7 summit in France last week by saying he would be prepared to meet his Iranian counterpart.

Le Drian said that Macron had "sensed that president Trump was open to softening the strategy of maximum pressure, to find a path that could allow a deal to be reached."