Officials from Britain, France and Germany began a meeting in Vienna -- the first since they launched a dispute process over alleged violations by Iran -- aiming to persuade the Middle Eastern country to fall back into line with the deal.

The other parties to the deal, China and Russia, are also taking part in the talks overseen by the European Union, with Russian Foreign Minister Sergei Lavrov saying on Twitter there was still a chance "to stop escalation before it is too late".

In its last announcement in early January, Tehran said it would no longer observe limits on the number of centrifuges used to enrich uranium.

It was its fifth step away from the deal since US President Donald Trump's withdrawal, and led to Germany, Britain and France triggering the dispute process on January 14.

The process spells out several steps, the last one of which is notifying the UN Security Council. UN sanctions would then automatically "snap back" after 30 days unless the Security Council voted to stop it.

- 'Far from a result' -

A diplomat who asked no to be named told AFP that no timetable had been fixed for solving the dispute, adding "we are still far from a result".

"We all want to save the JCPOA (Joint Comprehensive Plan of Action, as the deal is known) so that the inspectors can continue their work in Iran," the diplomat said, referring to the inspections by the International Atomic Energy Agency (IAEA).

The Vienna-based UN nuclear agency has been tasked with monitoring the deal's implementation and issues regular reports, the latest of which is expected within days.

Western diplomats believe Iran is highly unlikely to heed calls to come back into full compliance without substantial concessions in return -- such as an end to US sanctions or Europe taking measures to offset their economic impact.

But they hope the use of the dispute process will convince Iran not to make any more moves away from the deal, giving space for back-channel diplomacy aimed at bringing Washington and Tehran back into alignment.

The diplomat told AFP that Iran could also "at least freeze its uranium stocks" as a possible positive outcome of the current discussions.

At a major international security conference in Munich earlier this month, Iran's Foreign Minister Mohammad Javad Zarif said Tehran would be prepared to move back towards the deal if Europe provides "meaningful" economic benefits.

Europe has set up a special trading mechanism called Instex to try to enable legitimate humanitarian trade with Iran, but it has yet to complete any transactions and Tehran regards it as inadequate.

The renewed US sanctions have almost entirely isolated Iran from the international financial system, driven away oil buyers and plunged the country into a severe recession.