WASHINGTON — As a US-led coalition began strikes in Syria against the Islamic State this week, one target is likely to be American-made military equipment stolen by the terrorist group — a cache that has already played a key role in the conflict in Iraq.
Over the past six weeks, US warplanes destroyed at least three dozen US-made Humvees that were by stolen by the Islamic State. Earlier this week, Islamic State forces used Humvees to overrun an Iraqi army post.
The Islamic State's reliance on American-made equipment has highlighted concerns about plans to supply $500 million in high-tech weapons to the rebels known as the Free Syrian Army. Congress approved the plan but the majority of the Massachusetts delegation opposed it, with some basing their opposition partly on concerns about where the arms may end up.
Representative Joseph P. Kennedy III, a Brookline Democrat who voted against the proposal, said in an interview that he worries the allegiance of rebel groups would change, and whether "we would have some control over the end use of the arms provided."
Representative Michael Capuano, a Somerville Democrat who also opposed the measure, wrote bluntly to his constituents: "Let's not forget, a year ago many ISIS fighters would have been trained by the US as part of a 'trusted' force."
Some Republicans raised the concern as well, with Representative Tom McClintock of California, in a speech on the House floor, saying "the Islamic State is armed to the teeth — with American equipment."
US officials acknowledge that providing arms to the Syrian rebels poses a risk, but they insist it is a necessary one. Still, the extent of the loss of weapons has become a major issue as the Obama administration develops its strategy against the Islamic State.
"There will always be risk in a program like this, but we believe that risk is justified given the threat," Defense Secretary Chuck Hagel said in testimony Thursday before the House Armed Services Committee, where he pledged a more rigorous tracking system for the new arms. A Pentagon spokesman elaborated the next day that it would take up to five months to vet the Syrian rebels who eventually would receive arms.
The broader concerns were also raised in a Sept. 8 study by Conflict Armaments Research, a London-based nonprofit. It concluded: "Islamic State forces have captured significant quantities of US-manufactured small arms and have employed them on the battlefield."
The arms are believed to have found their way to the terrorist group through several means, including fighters that defected to its ranks, the black market, and as battle spoils.
There are also signs that weapons supplied covertly to the Syrian rebels have come under the control of the Islamic State or forces loyal to President Bashar Assad of Syria, who has been accused of ordering the killing of thousands in the country's civil war.
The Conflict Armaments Research study also found that antitank rockets from Croatia that were provided to the Free Syrian Army have also turned up in the arsenal of the Islamic State, commonly referred to by the acronyms ISIS and ISIL.
The Pentagon regularly reports how airstrikes in Iraq are pummeling the armored vehicles, artillery pieces, and tanks of the Islamic State, posting explosion-filled video as proof. What the nearly daily announcements don't highlight, however, is that much of the equipment being struck came from the United States.
Since the air assault began on Aug. 8, the Pentagon says, US aircraft have destroyed at least 37 Humvees, a rugged military transport vehicle that was provided in large quantities to the Iraqi government.
In one recent airstrike, US jets also took out a "mine-resistant ambush protected vehicle,'' or MRAP, a bulky armored US troop carrier designed, at about $600,000 apiece, to withstand roadside bombs in Iraq.
According to a number of local news media reports, the Islamic State has seized thousands of US-made weapons — including heavy artillery systems — and vehicles while overrunning Iraqi forces.
Representative Ted Poe, a Texas Republican, took to the House floor last week with large photos of some of the equipment that the militant group posted on the Internet.
"This is an American tank now in the possession of ISIS," Poe said. "This is a Humvee going to a parade. Now we want to arm Syrian rebels. Let's see how that's worked in the past."
While the recent examples of US weapons being diverted is difficult to fully track, the confirmed cases are mounting.
In July, a Pentagon inspector general report concluded that US forces had lost track of nearly half of the nearly 500,000 small arms, such as M16 automatic rifles, that were provided to the fledgling security forces of Afghanistan.
Some specialists, however, said the history of weapons "leakage" is not sufficient reason to refrain from assisting allied nations or groups.
"There are few massive cases of transfers," said Anthony Cordesman, a military specialist at the Center for Strategic and International Studies and longtime Pentagon adviser. "I can think of cases where [US enemies] benefited, but I can't think of a case where any movement really won because of access to US arms."