US PRESIDENT Donald Trump has abandoned the 2015 deal that limited Iran's ability to build nuclear weapons.
"The speed of the process is and has to be fast", Mogherini told a late night press conference.
Mogherini spoke after chairing emergency talks between interested parties in Brussels.
Foreign ministers of Germany, France and Britain met their Iranian counterpart in Brussels on Tuesday and tasked their experts to come up with measures for a meeting of their deputies in Vienna next week.
The group likened the state of the deal to a "relative in intensive care", according to Mogherini.
EU foreign policy chief Federica Mogherini said Tuesday that experts had already begun work on measures to get round United States sanctions, focusing on nine key areas including Iran's ability to continue selling oil and gas products, and how to protect European companies doing business in the country.
But he warned that Tehran expected to see progress towards the guarantees it wants "within the next few weeks".
European diplomats acknowledged that the EU support, however honest, risked looking hollow after Trump reimposed an array of wide sanctions last week on Iran that will hit European companies investing there.More news: Prosecutor DISMISSES invasion of privacy case against Gov. Greitens
Mogherini said European Union experts were aiming to come up with concrete proposals in the coming weeks on nine key issues including ensuring Iran could sell its oil and gas products and have access to global finance.
The EU wants to ensure exports to Iran, which have exploded since the deal was signed in 2013, can continue.
German exports to Iran totalled almost 3.0 billion euros in 2017, while French exports soared from 562 million euros in 2015 to 1.5 billion in 2017 and oil giant Total has pledged to invest some $5 billion in the South Pars gas field.
The European ministers also brought up in the talks their concerns over Iran's ballistic missile program and regional activities, diplomats said.
But preserving or increasing European investment in Iran in the face of the American sanctions is a hard challenge - especially since the White House has made clear to Europeans that their companies will not get exemptions from the sanctions, which Washington hopes will pressure Iran into a new set of negotiations.
"I cannot talk about legal or economic guarantees but I can talk about serious, determined, immediate work from the European side", Mogherini said.
"We know it's a hard task but we are determined to do it and we have started to work to put in place measures that help ensure this happens", she told reporters.
European Union leaders agreed to work with Iran to try to salvage the deal, but no immediate decisions were made.