Iraqi security forces hold an Islamist State group's flag, which they pulled down at the University of Anbar, in Anbar province, July 26, 2015. Reuters/Stringer

Decades of reckless arms trading and poorly regulated flow of weapons into Iraq have helped the Islamic State group, also known as ISIS, acquire its large and lethal arsenal, Amnesty International said in a report published Tuesday. The London-based human rights watchdog said that these weapons were being used to commit war crimes on a massive scale in Iraq and Syria.

The report, based on expert analysis of verified videos and images, claims that ISIS fighters have been using arms mainly seized from Iraqi military stocks. These weapons were manufactured and designed in more than two dozen countries, including Russia, China, the U.S. and EU states, according to the report.

"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," Amnesty researcher Patrick Wilcken said in the report. "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.”

According to the report, ISIS fighters acquired the huge amounts of internationally manufactured weapons when the Sunni-militant group took control of Mosul, Iraq’s second largest city, in June 2014.

Amnesty said the weapons have allowed ISIS to carry out a "horrific campaign of abuse," including "summary killings, rape, torture, abduction and hostage-taking -- often carried out at gunpoint."

The range and scope of the militant group’s large arsenal also reflects decades of "irresponsible" arms transfers to Iraq, the report said.

“This has been compounded by multiple failures to manage arms imports and to put in place oversight mechanisms to avoid improper end uses during the US-led occupation after 2003. Likewise, lax controls over military stockpiles and endemic corruption by successive Iraqi governments have added to the problem,” the watchdog's report claimed.

According to the report, ISIS has man-portable air defense systems (MANPADS), guided anti-tank missiles and armored fighting vehicles, as well as assault rifles like the Russian AK series and the US M16 and Bushmaster, as part of its advanced weaponry. Most of the conventional weapons being used by ISIS fighters date from the 1970s to the 1990s, when Iraq was engaged in a massive military buildup ahead of and during the 1980-1988 Iran-Iraq war.

“This shows again that arms export risk assessments and mitigation measures to unstable regions require a long term, root-and-branch analysis. This must include assessing if military and security units are capable of effectively controlling stockpiles and abide by international human rights and humanitarian standards,” Wilcken said, in the report.