The bloc wants to prevent European firms that invested in Iran after the deal, to comply with specific foreign-sanctions laws.
The European Union took formal steps on Friday to shield its firms from US sanctions on Iran as part of efforts to save the international nuclear deal with Tehran.
EU leaders meeting on Thursday in Bulgaria gave the European Commission, the bloc's executive arm, the all-clear amid a deepening rift with Washington.
The commission said on Friday it "launched the formal process to activate the blocking statute by updating the list of US sanctions on Iran falling within its scope."
No sanctions commitment
The commission said it hoped the statute would be in force before August 6 when the first batch of reimposed US sanctions take effect.
President Donald Trump last week pulled Washington out of the 2015 international deal with Iran to curb its nuclear programme in return for easing sanctions.
The statute, which the 28 EU member states and the European Parliament must endorse, is aimed at reassuring European firms that invested in Iran after the deal.
"The blocking statute forbids EU companies from complying with the extraterritorial effects of US sanctions," the commission said.
It also "allows companies to recover damages arising from such sanctions from the person causing them, and nullifies the effect in the EU of any foreign court judgements based on them," the executive added.
Brussels took steps on three other fronts to shore up the Iran deal, signed not just by the EU but EU members Britain, France and Germany, along with China and Russia.
The "blocking statute" is a 1996 regulation originally created to circumvent Washington's trade embargo on Cuba, which prohibits EU companies and courts from complying with specific foreign sanction laws.
However, the Cuba row was settled politically, so the blocking regulation's effectiveness was never put to the test, and its value may lie more in becoming a bargaining chip with Washington.
Since the US withdrawal, the remaining parties have all pledged to stick to the deal if Tehran respects its terms. Beijing and Moscow have also stepped up efforts to save the deal.
However, France seems pessimistic.
The head of France's business lobby in Iran said on Friday he feared a mass exodus of European firms following US' sanctions and that EU government efforts to protect their companies were unlikely to be enough.
"Sadly, what we can fear is a mass withdrawal of European firms," Matthieu Etourneau, managing director of the French Centre for Business in Tehran, said at a conference at the French Institute for International Affairs.
"The political pro-activeness of governments is good, but we have the feeling that it will not be enough to ensure companies keep their activities in Iran."
French energy group Total, which arguably signed the most symbolic contract between Iran and Western powers since 2016, said on Wednesday it might quit its multi-billion-dollar gas project in Iran unless it secured a waiver from the sanctions.
Tehran has warned it was ready to resume no-holds-barred "industrial-scale" uranium enrichment unless Europe could provide solid guarantees to preserve Iran's economic benefits under the deal.
During talks in Brussels on Tuesday, Iran's foreign minister Mohammad Javad Zarif said efforts to save the deal were on the "right track."
On other fronts, the commission moved on Friday to remove hurdles for the European Investment Bank (EIB) to finance activities outside the EU, such as in Iran.
It said the move will "allow the EIB to support EU investment in Iran," particularly involving small and medium-sized companies.
The commission on Friday called for doing more to help Iran's energy sector and small and medium-sized companies, as part of "confidence-building measures."
It added that EU energy and climate commissioner Miguel Arias Canete would travel to Tehran at the weekend.
The commission said it was also encouraging EU member states to explore the idea of "one-off bank transfers" to the Central Bank of Iran.
The approach, it said, could ensure Tehran receives its oil-related revenues if US sanctions target EU firms active in oil transactions with Iran.
In pulling out, Trump complained the nuclear deal does nothing to stop Iran's ballistic missile programme or its interference in conflicts across the Middle East from Syria to Yemen.