Frank-Walter Steinmeier said Iran's decision to resume research on the nuclear fuel cycle cannot remain "without consequences."
Steinmeier said that Iran's decision to break the seals on equipment needed to enrich uranium "would breach the agreements we made in Paris with France, Great Britain and Iran itself."
Germany, along with France and Britain, makes up the "troika" of European countries engaged in negotiating with Tehran to ensure that Iran's nuclear program is used for peaceful purposes only.
The West has warned that Iran's move to restart fuel-enrichment research could seriously jeopardize the negotiations.
Iran says it will not give up its right under international treaties to develop an expertise in developing the nuclear fuel cycle.
It has said the research is being carried out under the supervision of the International Atomic Energy Agency.