The United States has expressed concern that Iranian cargo planes overflying Iraq may be shipping weapons to Syria.
"We are concerned about the overflight of Iraq by Iranian cargo flights headed to Syria," State Department spokeswoman Victoria Nuland told reporters on Friday, adding that the U.S. and other countries were working with the Baghdad government to be "absolutely sure about any cargo that's overflying its territory."
Nuland said Iranian arms exports are subject to a UN arms embargo.
"Any arms sent to the Syrian regime at this time would obviously be used in the brutal repression that the regime is exacting on its own people," Nuland said.
The UN says more than 8,000 have died since anti-government protests erupted a year ago.
Earlier on Friday, Iraqi Prime Minister Nouri al-Maliki issued a statement saying Iranian cargo flights were transporting only humanitarian aid.
"Iraq does not allow its land or its skies to be a passage for weapons in any direction, and from any source," Maliki said.
The U.S. has made "several requests" in the past few months to Baghdad, to either stop Iranian flights, or allow the aircraft to be inspected, the Washington Times said on Thursday, citing an unnnamed U.S. official.
On Tuesday, an Iranian cargo plane heading to Syria's second city of Aleppo was told to land in Turkey amid concerns that it may be carrying arms. It was allowed to leave after search teams found that it was transporting 150 tons of food.
Meanwhile, the UN and Arab League envoy on Syria, Kofi Annan, said on Friday international teams would visit Damascus this weekend to "pursue the discussions on the proposals we left on the table."
Annan also called on the UN Security Council to speak with "one voice" to end the violence.