The deal over Iran's nuclear programme has begun to unravel since President Donald Trump announced the United States was withdrawing from the agreement last year and reimposing sanctions.

Iran has been pushing the European parties to the deal -- Britain, France and Germany -- to adhere to their commitments under the agreement despite US pressure.

British authorities seized a tanker carrying Iranian oil off its territory Gibraltar on July 4, a move Spain's foreign minister said was carried out at the request of the United States.

"They (the European parties) have set out their commitments and announced them, they (include) the sale of Iran's oil, the transportation of Iran's oil, and the return of Iran's oil income," said Iranian Foreign Minister Mohammad Javad Zarif.

"It is clear that today's tensions and problems are due to America's economic terrorism and Europe's inability to fulfil its commitments which means going along with America's economic terrorism," he said, quoted by state media.

Zarif's remarks come after a meeting in Vienna on Sunday of the remaining parties to the nuclear deal -- the three European nations plus China and Russia.

In remarks broadcast on state television, the top Iranian diplomat described the talks as "challenging".

"We raised our stance and the importance of the fulfilment of the commitments of other parties to the JCPOA, in particular European countries," he said, referring to the deal by its formal name, the Joint Comprehensive Plan of Action.

- Europeans 'must have courage' -

"We clearly explained to them that these commitments that have been raised have not been implemented and that INSTEX... still isn't fully operational," said Zarif.

INSTEX was a mechanism set up by Britain, France and Germany to facilitate trade with Iran in the face of US sanctions.

"It should not be the case that INSTEX becomes a tool for implementing America's orders," the foreign minister said.

"INSTEX must be considered as a European measure.

"They (the Europeans) must have the courage to act according to their commitments and not according to America's demands," he said.

One year after the US pullout, Iran said in May it would begin scaling back its commitments, and it has since started increasing its stockpile of enriched uranium and the level of enrichment beyond the deal's limits.

Zarif said Iran was ready to take a third step to reduce its commitments under the deal unless the remaining parties fulfil theirs, as they reiterated in Vienna.

"Now we'll have to see how they are going to act," he said.

"But in the current circumstances and as long as necessary measures are not taken, the Islamic Republic of Iran's third step will certainly be operational."