Iraq is unable to stop its neighbor Iran transferring weapons to Syria through its airspace, Iraqi Foreign Minister Hoshyar Zebari said in a newspaper interview published on Saturday.

"We reject and condemn the transfer of weapons through our airspace and we will inform the Iranian side of that formally. But we do not have the ability to stop it," he told pan-Arab daily Asharq al-Awsat.

Iran is the main ally of Syrian President Bashar Assad, who is fighting mostly Sunni rebels in a civil war in which more the opposition say more than 100,000 people have been killed.

The United States, which wants Assad to relinquish power, has warned Iraq not to allow Iranian weapons flights to cross its airspace into Syria. But Zebari said he had told Western countries that if they wanted to stop such flights, they had to help do so themselves.

"If you imagine these flights breach United Nations Security Council resolutions banning weapons imports and exports from Iran... I invite you in the name of the government to help us stop these flights across Iraqi airspace," he said.

He said random checks by Iraq on Iranian aircraft bound for Syria since September had only found non-lethal aid like medicines and food.

Iraq is dominated by Shi'ite Muslim parties with close ties to Tehran, but Zebari said his country was committed to neutrality in Syria. Two years of fighting there risks further destabilizing an already fragile Iraq as Shi'ite and Sunni fighters cross the long border between the two states.

Zebari said Iraq does not provide any weapons or money to Syria and, despite requests from Damascus, does not sell it crude oil at preferential rates.