France, Germany and Britain are leading European efforts to save the landmark 2015 deal to curb Iran's nuclear programme, which was thrown into crisis in May when US President Donald Trump withdrew and reimposed sanctions.

Critics of the accord -- which was also signed by Russia and China -- say it is too narrowly focused on the nuclear issue and does nothing to curb Iran's meddling in regional conflicts or its programme to develop conventional ballistic missiles.

The European Union is trying to find ways to maintain oil and banking ties with Iran after the second phase of US sanctions kicks in in November, and Le Drian said it was important to find ways to keep the Islamic republic trading.

But he warned that Tehran must keep up its side of the deal, and be prepared to expand talks.

"Iran must respect the fundamentals of the JCPOA (nuclear deal) and I think that is the case," Le Drian said as he arrived for a meeting of EU foreign ministers in Vienna.

"But Iran cannot avoid discussions, negotiations on three other major subjects that worry us -- the future of Iran's nuclear commitments after 2025, the ballistic question and the fact there is a sort of ballistic proliferation on the part of Iran... and the role Iran plays to stabilise the whole region.

"We must talk about these three subjects, Iran must be aware of this and that's the message I send to them from Vienna."

Despite European pledges to keep providing Iran with the economic benefits it received from the nuclear deal, many major companies have already pulled out of the country for fear of US penalties.