The study by the London-based small-arms research organisation Conflict Armament Research documented weapons seized by Kurdish forces from militants in Iraq and Syria over a 10-day period in July.
The report said the jihadists disposed of "significant quantities" of US-made small arms including M16 assault rifles and included photos showing the markings "Property of US Govt".
It also found that anti-tank rockets used by IS in Syria were "identical to M79 rockets transferred by Saudi Arabia to forces operating under the Free Syrian Army umbrella in 2013".
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The rockets were made in the then Yugoslavia in the 1980s.
Islamic State is believed to have seized large quantities of weapons from Syrian military installations it has captured, as well as arms supplied by the United States to the Iraqi army after it swept through northern Iraq in recent weeks.