"We are extremely concerned at Iran's announcement that it has started uranium enrichment above the limit of 3.67%," a German foreign ministry spokesman said.
"We strongly urge Iran to stop and reverse all activities inconsistent with its commitments."
The spokesman added that Germany was in touch with other European participants in the non-proliferation deal - Britain and France - to decide on the next steps. Iran blames the US, which withdrew from the deal, for the latest tensions.