U.S. officials with access to the latest U.S. intelligence on Iraq told Fox News it "appears likely/probable" that U.S.-made Stinger missiles have fallen into the hands of Sunni insurgents.

It is possible that the Islamic State of Iraq and Syria (ISIS) fighters acquired them from army bases they have taken over in recent days, the sources said.

The Stinger missile is a shoulder-fired surface-to-air weapon that is used against aircraft.

As ISIS forces have advanced through Iraq, concerns have increased that more U.S.-made weaponry could fall into the hands of the radical group.

Iraqi intelligence officials said ISIS fighters managed to take control of two big weapons depots late last week holding some 400,000 items, including AK-47 rifles, rockets and rocket-propelled grenades, artillery shells and mortars. A quarter of the stockpiles were quickly sent to Syria in order to help the group's comrades there, they said.

Also last week, according to a report from the West Point Combating Terrorism Center, the ISIS "now possesses scores of Iraqi military equipment originally provided by the United States, from Humvees and cargo vehicles to small arms."

Fox News' Jennifer Griffin, Jonathan Wachtel and The Associated Press contributed to this report.