Iran's acceleration of its enrichment of uranium to near weapons-grade breaches restrictions imposed in a 2015 nuclear deal, Germany said on Wednesday, urging Tehran to return to negotiations with a constructive approach. 

"Iran has no plausible civilian justification for these steps, and instead gains military knowledge and skills," a Foreign Ministry spokesman told a news conference. "We urge Iran to return to the negotiating table with a constructive stance." 

The UN atomic watchdog said in a report on Tuesday seen by Reuters that Iran had accelerated its enrichment of uranium. 

Iran's work on enriched uranium metal has angered Europe's three top powers and the United States because that technology, and knowledge of how to produce it, can be used to make the core of a nuclear bomb, Reuters reported on Tuesday. 

Iran insists its aims are entirely peaceful and it is developing a new type of reactor fuel.