The report published onTuesday states that poorly regulated movement of weapons in Iraq as well as "lax controls on the ground" have allowed extremist group Isis to stockpile a dangerous amount of arms.
Amnesty's findings were based on expert analysis of thousands of videos and images, showing that the weapons Isis uses come from Germany as well as Russia, China and the United States.
“The vast and varied weaponry being used by the armed group calling itself Islamic State is a textbook case of how reckless arms trading fuels atrocities on a massive scale,” Patrick Wilcken, researcher on arms control, security trade and human rights at Amnesty International, said in a statement.
Isis seized most of their weapons from the Iraqi army, though they may have also acquired some through battlefield capture, illicit trade or defections of Iraqi and Syria soldiers, Amnesty reports.
When Isis took control of Iraq's second-largest city, Mosul, in June of last year, the group obtained a huge supply of internationally produced weapons from the Iraqi military.
Iraq's weapon stockpile included arms from the Iran-Iraq war between 1980 and 1988 in which 34 different countries provided the country with weaponry, including West Germany. Additional equipment has also been supplied over the past decade.
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The extremists also have collections of German-produced machine guns, some of which may have been captured from Kurdish forces. A German-made handgun features in an Isis execution video and Franco-German designed missiles are also reported to be in their arsenal.
The German government recently approved a mission to Syria involving up to 1,200 troops. German reconnaissance aircraft were expected to begin flights over Syria this Monday.