She announced the European Union and Iran have launched "expert discussions" on measures needed to save the deal thrown into doubt by Donald's Trump's decision to withdraw the United States last week.
Mogherini says "there was awareness of the urgency" and the four ministers agreed to find some solutions within weeks.
"If I can use the metaphor that some raised around the table, we all have a relative in intensive care and we all want to get him or her out of intensive care as soon as possible", she said. These include protecting European companies dealing with Iran from USA sanctions, which in practice would be very hard, allowing the European Investment Bank to invest there and coordinating euro-denominated credit lines from EU states.
British, French and German foreign ministers vowed to try to keep Iran's oil and investment flowing after a meeting with their Iranian counterpart on Tuesday but admitted they would struggle to provide the guarantees Tehran seeks. "We see the same determination from the Iranian side". "A lot depends on what we can do within the next few weeks".
"If we want to save this deal, we know that the sooner we manage to do it the better and the easier it will be".
Trump denounced the accord, completed under his predecessor Barack Obama, because it did not cover Iran's ballistic missile program, its role in Middle East conflicts or what happens after the deal begins to expire in 2025. "But we think we can tackle those in other ways, outside the JCPOA".
Mogherini agreed at the end of a long day of diplomatic meetings.
Iraqi Election Frontrunner Moktada al-Sadr Courts Partners to Govern
Sadr has led two uprisings against USA forces in Iraq and is one of the few Shia leaders to distance himself from Iran. Voter turnout was at a low 44 percent, 15 percent lower than the turnout in the 2014 parliamentary elections .
"It's not right to cancel this agreement in this situation now", she said.
Talks are already underway between the EU and Iran about how to maintain trade amid the reintroduction of United States sanctions which could see European companies penalised.
"We have to be realistic about the electrified rail, the live wire of American extraterritoriality and how [it] can serve as a deterrent to business", Johnson told reporters.
Prime Minister Theresa May will be present when talks on the issue continue tomorrow evening at an European Union leaders summit in Sofia, Bulgaria.
It is unclear how well the measure could be enforced, given that big multinationals are likely to be doing more business in the US than they are in Iran and may be unwilling to compromise that market access.
The EU's top energy and climate official, Commissioner Miguel Arias Canete, is heading to Iran on May 18-21 for talks on energy cooperation, a symbolic gesture from the EU that it wants to stay engaged despite the US withdrawal.