The remaining parties to the Iran nuclear deal do not see Tehran's breaches as significant non-compliance and have not indicated any intent to trigger the accord's dispute mechanism, the European Union's foreign policy chief has said.

"For the time being, none of the parties to the agreement has signalled their intention to invoke this article, which means that none of them for the moment, for the time being with the current data we have had in particular from the IAEA, that the non-compliance is considered to be significant non-compliance," Federica Mogherini said.

She was speaking at a news conference following an EU foreign ministers meeting in Brussels.

The ministers held crisis talks on saving the beleaguered Iran nuclear deal as Britain warned the "small window" for success was closing and Tehran issued fresh threats of restarting its atomic programme.

Tensions in the Gulf have soared since last year, when the United States 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 today.

But Iran piled fresh pressure on Europe, demanding concrete measures to give it relief from US sanctions and threatening to return its nuclear programme to where it was before the curbs imposed by the 2015 deal.

British Foreign Secretary Jeremy Hunt - who held phone talks with his US and Iranian counterparts at 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," Mr Hunt told reporters.

Britain, France and Germany - the three European parties to the deal - yesterday issued a joint statement saying they were "extremely concerned" by Iran's recent breaches as well as by US sanctions, and called for dialogue to resolve the crisis.

Iran has repeatedly warned it could leave the deal unless the remaining parties bypass US sanctions to deliver the promised economic benefits and today threatened to take its nuclear programme back to its pre-deal status.

"If the Europeans and the Americans don't want to carry out their duties... we will decrease our commitments and... reverse the conditions to four years ago," said Iran's atomic energy agency spokesman Behrouz Kamalvandi, quoted by IRNA state news agency.

The US has vowed to pursue its "maximum pressure" campaign against Iran, but Spanish Foreign Minister Josep Borrell - lined up to be the next EU diplomatic chief - warned its strategy is only "strengthening the most radical" elements in the Islamic republic.

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 helping the Iranian economy while 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 in Brussels.

The Iranian foreign ministry warned in a statement that its compliance with the deal was "rooted in the principle of reciprocity" and demanded Europe come up with "practical, effective and responsible decisions".

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.

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

But the Iranian foreign ministry statement branded the European expectations "unrealistic", demanding a quid pro quo for undoing its recent breaches of the deal - including exceeding a 300kg limit on enriched uranium stockpiles.

"Any expectation for Iran to return to the conditions prior to May 8, 2019 without proof of any political will and practical ability of the European parties" to ensure sanctions relief "is an unrealistic expectation", it said.

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, Mr Hunt said.