The 2015 deal saw the lifting of sanctions on the Islamic Republic in return for curbs on its nuclear programme verified by the International Atomic Energy Agency (IAEA). Washington has since told countries they must stop buying the OPEC producer's oil from November 4 or face financial consequences.
The other partners so far appear powerless to stop their countries' companies pulling out of Iran for fear of United States penalties.
The top Iranian diplomat said the participants at the Vienna meeting agreed to take "good measures" to save the JCPOA.
The plan to shut down the critical shipping lane is said to be endorsed by Iranian President Hassan Rouhani, who, along with other top Iranian officials, has been working on contingency plans to combat US economic sanctions on Tehran following President Donald Trump's decision to walk away from the nuclear agreement.
German Foreign Minister Heiko Maas, French Foreign Minister Jean-Yves Le Drian, Chinese Foreign Minister Wang Yi, European Union High Representative for Foreign Affairs and Security Policy Federica Mogherini, Iran's Foreign Minister Mohammad Javad Zarif and Russian Foreign Minister Sergei Lavrov attend a meeting in Vienna, Austria July 6, 2018.
They said they remained committed to the 2015 accord and to building up economic relations with Iran, including "the continuation of Iran's export of oil and gas" and other energy products.
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The tour, which aimed to seek guarantees from European powers over the 2015 nuclear deal after the U.S. unilaterally pulled out in May, was overshadowed by news of the arrests.
She added that the participants will work on the issues such as "the encouragement of further investments in Iran, the protection of economic operators for their investments and other commercial and financial activities in or in relation to Iran, the bringing together of private and public sector experts, the practical support for trade with and investment in Iran, the protection of companies from the extraterritorial effects of USA sanctions". He promised to reinstate the previous anti-Iranian sanctions and even impose tighter ones. In a phone conversation with German Chancellor Angela Merkel on Thursday, Iranian President Hassan Rouhani described as "disappointing" the package of European proposals to save the Joint Comprehensive Plan of Action on the Iranian nuclear issue.
Iran's economy is already suffering from the sanctions that Washington re-imposed after walking away from the nuclear agreement, and the USA has threatened to punish companies from other nations that continue doing business with Iran.
Iran, which strongly denies ever seeking to build a nuclear bomb, has warned it could resume uranium enrichment for civilian purposes if the deal collapses. Several major firms - including France's Total and Peugeot, and Russia's Lukoil - have said they are preparing to leave. Rouhani called for a transparent operational program with a specific timetable to compensate for the United States' withdrawal from the agreement.
The rial currency has fallen, prices have risen and the country has been hit by street protests and strikes. He added that the commission needs to develop ways keep commercial and economic ties between Iran and other parties to the agreement, which "would not depend on the USA whims".