A British report has confirmed that ISIS, the Islamic State militant group, stole U.S.-made weapons from Syrian rebels and Iraqi government forces earlier this year and successfully transferred them between the two countries, showing a unexpectedly high level of logistical efficiency.

The Conflict Armament Research group worked with Kurdish forces in northern Iraq to document weapons recovered after skirmishes with ISIS.

"They are transporting these weapons in batches, and have a solid organizational approach to moving these weapons around," CAR researcher Shawn Harris told the Washington Post.

According to the report, the weapons that went missing from Iraq turned up in Syria just two weeks later, proving that a competent logistical network does exist within the ISIS-declared Islamic State. 

The weapons originally fell into the hands of ISIS after the group attacked and took over Iraqi military compounds. Iraq's armed forces had been equipped with American-made weapons, vehicles and equipment after U.S. troops withdrew from Iraq in 2011. The weapons, which are now being used by Kurdish forces after they took back key military positions in Mosul, include assault rifles and a number of other small arms, and some Humvee vehicles.

“The team worked alongside Kurdish People’s Protection Units (YPG), which operate primarily in Syria, and Kurdish Regional Government Peshmerga Forces in Iraq, to document captured IS weapons,” the report said.

ISIS also took anti-tank rockets that were originally given to the Free Syrian Army rebels by Saudi Arabia in 2013.

It was also noted that some of the weapons’ ID numbers had been welded over in an attempt to hide their origin.