France, Germany and Britain have set up a mechanism for non-dollar trade with Iran to avert US sanctions, although diplomats acknowledge it is unlikely to free up the big transactions that Tehran says it needs to keep a nuclear deal afloat.
Washington's major European allies opposed last year's decision by US President Donald Trump to abandon the 2015 deal, under which international sanctions on Iran were lifted in return for Tehran accepting curbs on its nuclear programme.
Iran has threatened to pull out of the deal unless the European powers enable it to receive economic benefits. The Europeans have promised to help companies do business with Iran as long as it abides by the deal.
Washington says that, although Iran has met the terms, the accord was too generous, failing to rein in Iran's ballistic missile programme or curb its regional meddling.
New US sanctions have largely succeeded in persuading European companies to abandon plans to invest in Iran.
The European trade vehicle was conceived as a way to help match Iranian oil and gas exports against purchases of EU goods. However, those ambitions have been toned down, with diplomats saying that, realistically, it will be used only for smaller trade, for example of humanitarian products or food.
"It won't change things dramatically, but it's an important political message to Iran to show that we are determined to save the JCPOA (Iran deal) and also to the United States to show we defend our interests despite their extraterritorial sanctions," one European diplomat said.
The EU has spent months preparing the system and it will take several months more to become operational.
Iranian Deputy Foreign Minister Abbas Araqchi said the mechanism was a good first step.