And it is far from clear that Mr. Trump would go along with any bailout: His national security adviser, John R. Bolton, has made it clear that he opposes such an agreement. So does Prime Minister Benjamin Netanyahu of Israel, who opposed the 2015 deal and pressed Mr. Trump to make good on his campaign promises to abandon it.

“This is precisely the wrong timing to hold talks with Iran,” Mr. Netanyahu said last week.

Without Trump administration support for the deal, it is not clear whether European banks would risk American sanctions by extending credit to Tehran or whether the credit might be extended by the European Central Bank, or France’s central bank, which would be more difficult for Washington to sanction.

If the talks, which French and Iranian officials began in Paris on Monday, are successful, Iran would return to the restrictions negotiated with the Obama administration four years ago. Mr. Trump abandoned that agreement in 2018, and the Iranians have stepped up pressure on Europe by gradually discarding the nuclear-production restrictions to which it had agreed.

“It was pretty much a technical discussion, and it went pretty well, on the whole,” a spokeswoman for the French foreign ministry said on Monday. But she gave no details and would not confirm that the centerpiece of the negotiation was the $15 billion letter of credit.

Iran has said that if the talks fail, it will escalate its nuclear activity starting Friday.

While it has not said what actions it might take, Iranian officials have hinted they are planning to raise their enrichment of uranium to 20 percent purity — a level that takes them significantly closer to bomb-grade fuel.