"What we are hearing currently out of Tehran is not a positive contribution, particularly the development in Natanz," said German Foreign Minister Heiko Maas ahead of talks in Vienna on Tuesday.

A day after Tehran said it started up advanced uranium enrichment centrifuges in Natanz in breach of the 2015 nuclear deal, it said the site was hit by a power outage that it labelled "terrorism" and blamed on Israel.

Maas said the negotiations in Vienna "will not be easy but until now, there has been a constructive spirit" shown by participants.

At the same time, he noted that the situation in Natanz will take on a "particular meaning" and could give clues to what extent what is discussed in Vienna would then be implemented by Tehran.