The United States wants Iraq to stop Iran from using Iraqi airspace to ship weapons to Syria.
Iranian President Mahmoud Ahmadinejad is one of the last major suppliers of military intelligence and weapons to embattled Syrian President Bashar al-Assad.
U.S. officials say Iran's support for the Syrian government, along with help from Russia, is one of the biggest reasons that President Assad has survived a year-long uprising. Most of those Iranian weapons are flown to Syria through Iraq, which also supports Assad.
State Department spokeswoman Victoria Nuland says Baghdad should prevent the shipments.
"We are making the point that any export of arms or related materials from Iran, frankly, to any destination would be a violation of U.N. Security Council Resolution 1747, and that 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," said Nuland.
Foreign policy analyst Malou Innocent says Iran enhances its regional power by supporting President Assad.
"Assad's government has been seen as a sort of an arm of the Iranian government in terms of assisting proxy forces in Lebanon through Hezbollah and also with Hamas," said Innocent.
In Lebanon, supporters of Sunni cleric Ahmad al-Assir oppose Iran's involvement in Syria because they say it undermines Lebanese security.
''When we started our movement, many people told us, 'We agree there is injustice in Syria, but we are afraid,'" al-Assir noted. "When I asked them why, they say they are afraid of the allies of Syria, like Hezbollah."
Iran says clerics such as al-Assir and Saudi calls to arm Syria's opposition show the need for Tehran's Shia leaders to back President Assad against a Sunni-led power grab.
"There has certainly been a sharpening division between Shia and Sunni blocks within the region. We have seen that now play out not only in Libya and in Syria and Iran, but also in Bahrain and in Yemen," added Innocent.
Iraqi officials say they have no interest in helping Iran prolong Syria's uprising as Syrian refugees from the conflict come to Iraqi Kurdistan. U.S. officials say senior members of the government in Baghdad have expressed a commitment to ensure that Iraqi territory and airspace are not used as a transit point for Iranian weapons to Syria.