A European payment system designed to circumvent US sanctions on Iran will be ready soon, Germany announced on Monday.
German Foreign Minister Heiko Maas met Iranian President Hassan Rouhani and Zarif in Tehran as part of European efforts to salvage the historic nuclear pact and defuse rising US-Iranian tension.
Iran and Germany held "frank and serious" talks on saving the 2015 nuclear deal with world powers, Zarif told a joint press conference.
"Tehran will co-operate with EU signatories of the deal to save it," Zarif said.
Iraq's president calls on neighbours, allies for Iran's stability
Maas earlier said the payment system, known as Instex, will soon be ready to go after months of work.
"This is an instrument of a new kind so it's not straightforward to operationalise it," he said, pointing to the complexity of trying to install a totally new payment system.
"But all the formal requirements are in place now, and so I'm assuming we'll be ready to use it in the foreseeable future," added Mass about the system for barter-based trade with Iran.
A cautious thaw in relations between Tehran and Washington set in in 2015 when Iran struck a deal with six big powers limiting its nuclear activity.
But tensions with the US have mounted since President Donald Trump withdrew Washington from the accord in 2018 and re-imposed sweeping sanctions.
Iran has criticised the European signatories of its 2015 nuclear deal for failing to salvage the pact after Trump pulled the US out.
Foreign ministry spokesperson Abbas Mousavi said on Monday that Iran will consider negotiating with the US if it halts "economic terrorism" against the Iranian people.
"We will think about the rhetoric and words when they are accompanied with actions and change of attitude. The best example would be a decrease in pain and suffering of the Iranian people caused by the illegal and targeted pressure by the US government against the Iranian nation," Mousavi said.
The UN atomic watchdog's chief Yukiya Amano expressed concern about rising tensions over Iran's nuclear programme, calling on Monday for dialogue to ease them.
In an effort to protect at least some of Iran's economy and keep alive the big-power nuclear deal that Washington is about to quit France, Britain and Germany have set up Instex, in the form of a special-purpose vehicle.
The three European Union members maintain the nuclear pact remains the best way to limit Iran's enrichment of uranium, a potential pathway to the development of nuclear weapons, and have been trying to get Iran to keep its commitments under the deal to cut back its nuclear programme.
On a weekend stopover in Iraq en route to Tehran, Maas warned of the dangers that conflict with Iran posed for the entire Middle East, saying the Europeans were convinced it was worth trying to keep the nuclear pact with Iran.
In Abu Dhabi, he also sounded a warning to Iran.
"We are not prepared to have a discussion on 'less for less'," Maas told reporters, referring to Iran's decision to reciprocate for the re-imposition of sanctions.
Missile and military capabilities
Last month, Iran scaled back some commitments under the 2015 deal and warned in 60 days it would resume refining uranium to a higher fissile degree than that permitted by the accord if Europe failed to shield its trade from US sanctions.
Iran has always said its nuclear activity is peaceful and refuses to put its missile and military capabilities on the negotiating table, as the Trump administration has demanded.
Maas co-ordinated his trip with France and Britain, and also discussed it with US Secretary of State Mike Pompeo.
In addition to Germany's visit, Japan's Prime Minister Shinzo Abe arrives on Wednesday in Iran on a rare diplomatic mission, hoping to ease the tensions between Tehran and Washington.
Japan wants to lower the temperature, officials say, with Abe winning Trump's blessing for the mediation trip when the US leader visited Tokyo last month.
However, the visit by Japan's leader "faces substantial obstacles and is unlikely to bear fruit", said Tobias Harris, an analyst at Teneo consultancy group.