The American effort to stem the flow of Iranian arms to Syria has faltered because of Iraq’s reluctance to inspect aircraft carrying the weapons through its airspace, American officials say.

The shipments have persisted at a critical time for President Bashar al-Assad of Syria, who has come under increasing military pressure from rebel fighters. The air corridor over Iraq has emerged as a main supply route for weapons, including rockets, antitank missiles, rocket-propelled grenade and mortars.

Iran has an enormous stake in Syria, which is its staunchest Arab ally and has also provided a channel for Iran’s support to the Lebanese Islamist movement Hezbollah.

To the disappointment of the Obama administration, American efforts to persuade the Iraqis to randomly inspect the flights have been largely unsuccessful.