Tensions in the Gulf have soared since the United States last year pulled out of the 2015 deal and reimposed sanctions on Iran, hammering its economy and prompting Tehran to break limits on uranium enrichment and stockpiling.

The European Union is desperately trying to prevent the deal unravelling completely, seeing it as the best way to stop Tehran acquiring atomic weapons, and the issue was top of the agenda as ministers from the bloc met in Brussels.

British Foreign Secretary Jeremy Hunt -- who held phone talks with his US and Iranian counterparts over the weekend -- insisted "the deal isn't dead yet".

"Iran is still a good year away from developing a nuclear weapon. We think there is still some closing but small window to keep the deal alive," Hunt told reporters.

Britain, France and Germany -- the three European parties to the deal -- on Sunday issued a joint statement saying they were "extremely concerned" by Iran's recent breaches as well as by US sanctions

They called for dialogue to resolve the crisis.

- No 'less for less' -

Europe hopes to use a special trading mechanism called INSTEX to enable businesses to deal with Iran without using the US dollar or financial system, thereby avoiding Washington's sanctions.

But the system is complicated, no transactions have been finalised yet and it can for now only be used for humanitarian goods -- food and pharmaceuticals, for example -- and not Iran's crucial oil sector.

Iran accuses Europe of not doing enough, but the sweeping nature of the US measures has scared many major European businesses out of Iran despite Brussels' insistence that American sanctions do not apply in Europe.

"Iran has taken bad decisions in response to the bad decision of the United States to pull out of the deal and reimpose sanctions, whose extraterritoriality strikes at the economic advantages the country got from the deal," French Foreign Minister Jacques-Yves Le Drian said as he arrived for talks in Brussels.

EU ministers insisted Iran must return to respecting its obligations under the deal in full, rejecting a suggestion by Iranian President Hassan Rouhani that both sides could reduce their commitments.

"This is a very, very serious situation. We must make it clear once again, in clear language, that there is only a chance if Iran commits itself unreservedly to what is contained in the treaty," Germany's junior foreign minister Michael Roth told reporters.

Hunt agreed, saying the deal "has to be taken in its entirety".

The joint commission overseeing the accord, made up of ministers from the countries still in the deal -- the Europeans plus China, Russia and Iran -- will meet "very soon" to discuss Tehran's breaches, Hunt said.