Yas Air and the SAD Import Export Company were added to a growing Iran sanctions list for breaching an arms embargo ordered against the Islamic republic for its nuclear drive.
According to diplomats, Turkish authorities intercepted arms shipments organized by Yas Air to Syrian President Bashar al-Assad in 2011.
US ambassador Susan Rice said that Yas Air and SAD were "significantly involved in Iranian arms smuggling."
The two firms were "responsible for shipping ammunition, assault rifles, machine guns, mortar shells and other arms from Iran to Syria," Rice said in a statement.
The sanctions action orders an assets freeze against the firm and bans dealings with the companies.
The UN Security Council has ordered four rounds of increasingly tough sanctions against Iran over its refusal to halt uranium enrichment. Western nations say this is part of a drive for a nuclear bomb. Iran denies the charge.
Iran is also a key backer of the Syrian leader and the sanctions regime allows countries to inspect Iranian cargo deliveries to prevent weapons proliferation.
"We have long known that Iran smuggles weapons and provides military assistance to terrorists and extremist groups worldwide," said Rice.
"Iran has also repeatedly been caught providing weapons and military assistance to Syria, where the Assad regime has used them against Syrian civilians."