Decades of wars, reckless arms trading and lax control of weapons have allowed the Islamic State group (ISIS) to build a massive arsenal of weapons, many manufactured in the U.S., which the extremists are using "to commit war crimes and crimes against humanity on a massive scale in Iraq and Syria," human rights group Amnesty International said in a new 44-page report released Tuesday.

The report, "Taking Stock: The Arming Of Islamic State," draws on expert analyses of thousands of verified images of ISIS militants and concludes that the extremists are mostly using arms and ammunition captured from the Iraqi military and Syria rebels, much of which was manufactured and designed in 25 countries, including the U.S., China, Belgium, France, Russia and Germany.

"In the 1970s and 1980s at least 34 countries, led by Russia, France and China, irresponsibly transferred billions of dollars' worth of military equipment to Iraq," the reports says. "During the invasion and its aftermath, the US-led coalition's decision to disband the Iraqi army, estimated at around 400,000 personnel, meant that many tens of thousands of individuals returned home or went into hiding with their weapons."

Amnesty International also highlighted the U.S.'s more recent shipment of weapons to Iraq: "Between 2011 and 2013, the USA signed billions of dollars' worth of contracts for 140 M1A1 Abrams tanks, F16 fighter aircraft, 681 Stinger shoulder held units, Hawk anti-aircraft batteries and other equipment. By 2014, the USA had delivered more than US$500 million worth of small arms and ammunition to the Iraqi government."

ISIS was able to steal a large amount of weapons from the Iraqi military when the terror group overran a series of cities in northern and western Iraq. For example, upon capturing Mosul, Iraq's second largest city, the group commandeered more than 2,300 U.S. armored vehicles from the fleeing Iraqis, according to The Guardian.

Weapons supplied by the U.S., Saudi Arabia, Qatar and Turkey to rebel groups fighting to overthrow Syrian President Bashar al-Assad also routinely end up in the hands of Islamic extremists, when rebels defect or are captured. Sometimes, the U.S. accidentally airdrops weapons directly to ISIS, as The Washington Post reported last year.

"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," said Patrick Wilcken, researcher on arms control, security trade and human rights at Amnesty International. "Poor regulation and lack of oversight of the immense arms flows into Iraq going back decades have given IS and other armed groups a bonanza of unprecedented access to firepower."

The Amnesty report calls for all countries to stop sending weapons to the Syrian rebel groups as well as perform proper risk assessment prior to sending weapons to the Iraqi government, according to Newsweek.