Wary of US President Donald Trump's tough talk on Iran, the European Union is courting Tehran to signal to its commitment to a nuclear deal and holding out the promise of its economic payoff ahead of May 19 presidential polls, EU diplomats say.

Europe's energy commissioner is leading more than 50 European firms in a business forum in Tehran over the weekend - the latest sally to help knit new trade ties in the 16 months since Iran curbed its nuclear programme in exchange for relief from sanctions.