The air corridor over Iraq has emerged as a supply route for weapons for the beleaguered government of Syrian President Bashar al-Assad, including rockets, anti-tank missiles, rocket-propelled grenade and mortars.
Citing unnamed US officials, the New York Times newspaper said that to the disappointment of the administration of US President Barack Obama, American efforts to persuade the Iraqis to randomly inspect the flights have been largely unsuccessful.
Iran has an enormous stake in Syria, which is its staunchest Arab ally and has also provided a channel for the Islamic republic's support to the Lebanese Islamist movement Hezbollah, the report said.
According to the paper, Iran appears to have been tipped off by Iraqi officials as to when plane inspections would be conducted, thus helping Tehran avoid detection.
Iran's continued efforts to aid the Syrian government were described in interviews with a dozen US administration, military and congressional officials, the Times said.
"The abuse of Iraqi airspace by Iran continues to be a concern," the paper quoted one of the officials as saying.
"We urge Iraq to be diligent and consistent in fulfilling its international obligations and commitments, either by continuing to require flights over Iraqi territory en route to Syria from Iran to land for inspection or by denying overflight requests for Iranian aircraft going to Syria."