BERLIN, Aug 18 (Reuters) - 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 U.N. atomic watchdog said in a report on Tuesday seen by Reuters that Iran had accelerated its enrichment of uranium. Reporting by Paul Carrel Editing by Madeline Chambers Our Standards: The Thomson Reuters Trust Principles.