Weapons supplied by the United States and Saudi Arabia to opposition fighters often fell into the hands of ISIL, significantly enhancing the "quantity and quality" of the group's armaments, a new report alleges.
The number of weapons goes "far beyond those that would have been available through battle capture alone", according to the study by arms-monitoring group Conflict Armament Research (CAR) published on Thursday.
It analysed more than 40,000 items found on the battlefield including weapons, ammunition, and materials used to make improvised explosive devices. Some were acquired by ISIL through shifting alliances within the Syrian opposition.
The research said most weapons were looted from the Iraqi and Syrian armies, however, some were originally supplied by other nations involved in the conflict to Syrian opposition groups fighting against President Bashar al-Assad.
"Iraq and Syria have seen IS forces use large numbers of weapons, supplied by states such as Saudi Arabia and the United States, against the various international anti-IS coalitions that the two states support," said CAR.
"Evidence collected by CAR indicates that the United States has repeatedly diverted EU-manufactured weapons and ammunition to opposition forces in the Syrian conflict. IS forces rapidly gained custody of significant quantities of this materiel," it said.
About 90 percent of weapons and ammunition used by the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) originated in China, Russia, and Eastern Europe, with Russian-made weapons outnumbering those of any other country.
"These findings support widespread assumptions that the group initially captured much of its military material from Iraqi and Syrian government forces," said the report.
The monitoring group said Western-produced arms and ammunition represented the remaining 10 percent.
This information is "a stark reminder of the contradictions inherent in supplying weapons into armed conflicts in which multiple competing and overlapping non-state armed groups operate", said CAR.
The organisation reported last year that ISIL had been producing military-grade weapons in Iraq, and had standardised production across its self-styled caliphate.
The Syrian civil war has raged for more than six years. Hopes of finding a diplomatic resolution to the conflict suffered another blow on Thursday after the failure of the latest round of UN-sponsored talks.