Iran continued to violate international law in this respect because it was failing to adhere to UN Security Council resolutions, German government spokesman Ulrich Wilhelm said.
"There remains a cause for concern," the spokesman said, two days after a US intelligence assessment concluded that Tehran appeared to have suspended its nuclear weapons program in 2003.
At the same time, Wilhelm said an offer of cooperation with Iran remained in force if the government there agreed to halt enrichment, which some nations in the West fear is geared towards making a bomb.
"The findings (of the report) confirm what the German government has always said -- that diplomatic negotiations are promising," Wilhelm told a press conference.
"But you also know the reality that two UN Security Council resolutions have not been followed and that Iran has not yet stopped its uranium enrichment program."
Diplomatic pressure must therefore be maintained on Iran, along with a readiness to cooperate with the Islamic republic if it agrees to halt enrichment, Wilhelm added.
Report leads to calls for a halt to sanctions
The US report released Monday contradicted White House warnings that Iran was closing in fast on the ability to produce its own nuclear weapon.
UN Security Council members Russia and China have suggested the report lessens the need for further UN sanctions against Iran.
But Wilhelm said: "The Security Council is preparing a further resolution. It is Germany's strong belief that we should continue to work towards a diplomatic solution together with China, Russia, the European Union and the United States."
US President George W. Bush has said the report did not change his view that "Iran was dangerous, Iran is dangerous and Iran will be dangerous if they have the knowledge necessary to make a nuclear weapon."
Pressure works, claims Steinmeier
On Tuesday, Foreign Minister Frank-Walter Steinmeier said the US intelligence finding proves a united stance in the international community is needed.
He said the unity of the European Union, the United States, Russia and China had "evidently not failed to make an impression on the political leadership in Iran."
"I appeal to the Iranian leadership to continue cooperation with the International Atomic Energy Agency (IAEA) and to respond to the unanswered questions about their nuclear program promptly and comprehensively," he said.