ZURICH — Uncertainty over what US President Donald Trump will do with the Iran nuclear deal hung over a conference here on Europe-Iran business that showcased both progress as well as continuing internal Iranian obstacles to foreign trade and investment.

European diplomats have put forward a tough and united front against any US reimposition of nuclear-related sanctions at a time when the International Atomic Energy Agency has repeatedly confirmed Iranian compliance with the 2015 agreement. But European executives appear less confident about their ability to shield Iran deals from a potential resumption of US penalties if Trump decides not to certify that Iran remains in compliance with the deal later this month.

“We should be able to continue to work,” Philippe Delleur, a senior vice president of the French train manufacturer Alstom, said in response to a question from Al-Monitor about the impact of a possible return of US sanctions.

At the same time, Delleur conceded that any such US action would negatively affect long-term financing for investments such as a recent Alstom contract to build railway and subway cars in Iran.