On Tuesday, however, Iran announced that it was preparing its nuclear facilities to resume large-scale uranium enrichment and had built a factory for constructing advanced centrifuges should Europe fail to preserve the deal.

On Wednesday, the French foreign minister, Jean-Yves Le Drian, said that Iran’s announced intention to increase its enrichment capacity still left it within the limits set out in the agreement, but, he added: “It’s always dangerous to flirt with the red line.” So far, he told Europe 1 radio, the deal remained intact.

Iran’s leadership is divided, but some would prefer to stay in the deal even without the Americans, to avoid possible military strikes from Israel and from the United States and to try to upgrade the economy. But that prospect seems unlikely if European companies are denied a waiver from the “secondary” sanctions.

Iran negotiated the 2015 deal in order to get relief from those crippling secondary sanctions, which Washington is now reimposing and which would deny access to financial systems in the United States to any company doing business with Iran.

The Europeans have been discussing efforts to shield their companies or to persuade them to continue doing business in Iran, possibly by creating a separate financing arm or by extending loans from the European Investment Bank. But on Wednesday, the bank said it could not afford to ignore the United States sanctions, because it needed to maintain access to capital markets.

Also on Wednesday, the European Commission updated its blocking statute, a mechanism that bans companies in the European Union from complying with extraterritorial legislation adopted by a third country. The mechanism, which dates from 1996 and has never been used, would go into effect two months from now in the absence of opposition from member states or the European Parliament. But few analysts expect it to have much impact, if any, even if it is adopted.

It is very difficult to persuade large companies with interests in the much larger American market to violate the sanctions or to act as guinea pigs in a trans-Atlantic scuffle over legal mechanisms when there is no certain outcome.