Germany on Wednesday condemned "in the strongest terms" the twin attacks that killed 12 people in Iran.

Foreign Minister Sigmar Gabriel said "once again, unscrupulous criminals have killed many innocent people".

"I condemn in the strongest terms the attacks in the Iranian parliament building and the tomb of the revolutionary leader (Ayatollah Ruhollah) Khomeini," he said in a statement.

"We mourn with the people of Iran. We offer our condolences to families of the victims. We hope that the many injured will fully recover soon."

Twelve people were killed when gunmen and suicide bombers stormed Iran's parliament and the shrine of its revolutionary leader.

The assaults were claimed by the Islamic State group, the first time it has taken responsibility for an attack in Iran.