Major European powers have sought to keep Iran in a landmark worldwide nuclear agreement after President Donald Trump pulled the USA out of the pact and promised tough economic sanctions against Tehran.
It termed the sanctions as illegal and warned that if talks to rescue the accord fail, it would ramp up its nuclear program to a level more advanced than before. "We're not there, we're beginning the process", Zarif told reporters after the talks.
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.
"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".
The group likened the state of the deal to a "relative in intensive care", according to Mogherini. "A lot depends on what we can do within the next few weeks".
European Union leaders at a summit in Sofia are expected to discuss measures that the bloc might take to shield European companies from renewed USA sanctions on Iran.
"It's not right to cancel this agreement in this situation now", she said.
Iran is seeking assurances over the sale of its oil and gas to Europe, as well as its continued ability to access financial services.
The EU hopes to convince Iran to continue to respect the landmark nuclear pact.
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.
French exports to Iran doubled previous year to €1.5bn, while German exports rose by €400m to €3bn.
Mogherini said she couldn't give any legal or economic guarantees to Iran about the continuation of the deal.
Speaking ahead of the meeting, Boris Johnson said he wanted to "protect legitimate United Kingdom and European businesses, who may want to trade with Iran, who do want to trade with Iran, and who have great plans to do that".
"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.
The president of the European Council, Donald Tusk, announced that during the informal summit of the EU on the Balkans that will be held on May 17 in Sofia, Bulgaria, it is expected to agree on a common response of the 28 member states of the regional bloc on the JCPOA against the withdrawal of the United States from the agreement.