Decades of poorly regulated arms flows into Iraq as well lax controls on the ground have provided the armed group calling itself Islamic State (IS) with a large and lethal arsenal that is being used to commit war crimes and crimes against humanity on a massive scale in Iraq and Syria.
The report, 'Taking Stock: The arming of Islamic State', says that weaponry supplied by the US and UK following the first Gulf War was left behind in many cities and towns and — together with other arms supplied in the past decade — much of it has fallen into the hand of Daesh.
Amnesty states that weapons supplied by Britain, following the 2003 invasion of Iraq, have also been taken by the Islamic militant group.
The report says the vast majority of weaponry used by Daesh was taken from stocks previously owned by the Iraqi army, much of which was supplied from western sources.
"The UK was involved in supplying the Iraqi security services. In early 2007, China shipped approximately 20,000 assault weapons to the UK for onwards shipment to the Iraqi security services," the report found.
"Similarly, between March 2005 and December 2006, a variety of small arms and light weapons were exported from Bosnia and Herzegovina and Serbia to the UK, and then re-exported to Iraq.
"As a result of systematic failures at all points of the US-led coalition distribution process in Iraq, hundreds of thousands of weapons transferred by the US and other members of the coalition want astray."
"The legacy of arms proliferation and abuse in Iraq and the surrounding region has already destroyed the lives and livelihoods of millions of people and poses an ongoing threat. The consequences of reckless arms transfers to Iraq and Syria and their subsequent capture by IS must be a wake-up call to arms exporters around the world," 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," said Wilcken.
After taking control of Mosul, Iraq's second largest city, in June 2014, Daesh fighters acquired a windfall of internationally manufactured arms from Iraqi stockpiles. They included US-manufactured weapons and military vehicles, which they used to take control of other parts of the country, with devastating consequences for the civilian population in those areas.
The vast array of weapon types captured and illicitly acquired have enabled Daesh to carry out a horrific campaign of abuse. Summary killings, rape, torture, abduction and hostage-taking — often carried out at gunpoint — have forced hundreds of thousands to flee and become internally displaced persons or refugees.