"It only takes one stray MANPAD to sneak into Turkey and that would be a very bad thing," an unnamed US official told the LA Times.
After all, Washington has already learned the hard way what could happen to the US-made military equipment supplied to the militants. Last September, the Pentagon confirmed that some of the rebels vetted and trained by the agency transferred their weapons to al-Nusra.
Even if Syrian extremists do not receive MANPADs from the US, they could still buy the shoulder-launched surface-to-air missiles on the black market.
The Gulf states, particularly Saudi Arabia, are another possible source. Riyadh has long provided financial assistance, as well as weapons to those trying to overthrow Bashar al-Assad.
"There is definitively concern about it," an unnamed defense official told the Daily Beast. "If [MANPADs] were to be introduced, there is a real worry that [Daesh] would target the coalition and eventually target civilian aircraft."
Two Syrian military aircraft were reportedly downed using anti-aircraft missiles earlier this year. Syrian rebels acquired MANPADs from government stockpiles in the early months of the conflict. Additional missiles were reportedly provided by Qatar.
Syrian MiG-21 was downed by MANPAD shot near Kafr Nabudah on March 12, 2016 at 2.41 p.m. The aircraft was performing airspace control tasks— Минобороны России (@mod_russia) 13 марта 2016 г.
In recent weeks, Syrian rebels have increasingly pressed Washington and its allies to send anti-aircraft weapons in case the ceasefire, brokered by Russia and the US, does not hold. Meanwhile, Moscow has made every effort to help the fragile Syrian peace process move forward.