WASHINGTON (AP) — Secretary of State John Kerry told his Iraqi counterpart Thursday that Iraq must stop weapons from flowing through its airspace to arm the forces of embattled Syrian President Bashar Assad.

Iraq and Iran claim the flights are carrying humanitarian goods, but the U.S. is confident that air shipments — along with material being trucked across Iraqi territory from Iran to Syria — are helping Assad’s regime cling to power by increasing its ability to strike Syrian rebels and opposition figures demanding his ouster. The issue has long been a source of contention between the U.S. and Iraq, and Kerry raised it again during a meeting at the State Department with Iraqi Foreign Minister Hoshyar Zebari.