WASHINGTON—The Obama administration sanctioned an Iranian airline for allegedly ferrying machine guns and munitions into Syria to help President Bashar al-Assad put down a rebellion against his rule.

The shipments, according to U.S. officials, are part of an operation headed by Iran´s elite military unit, the Islamic Revolutionary Guard Corps, to help bolster the Syrian regime.

Iranian and Syrian officials have repeatedly denied that Tehran is supplying arms to the Assad regime. They have also accused the U.S. and its Arab allies of fomenting the revolution against the Syrian government.

The Treasury on Tuesday also sanctioned three commanders from the IRGC´s international arm, the Quds Force, for their alleged involvement in arms smuggling, particularly into Africa. The U.S. believes the Quds Force is using shipping companies in African countries, such as Nigeria, to move weapons into conflict areas, including Gambia. Shipments of arms apparently bound for Gambian rebels were seized by Nigerian secret service agents in late 2010, prompting Gambia to cut off diplomatic relations with Tehran.

"As the Iranian regime exports its lethal aid and expertise to foment violence in Syria and Africa, Treasury will continue to expose the officials and companies involved," the Treasury Department´s pointman on counterterrorism, Undersecretary David Cohen, said in a statement Tuesday.

The sanctions bar U.S. citizens from any dealings with the Iranian airline, Yas Air, or the Quds Force commanders, freeze any assets they may have under U.S. control, and subject foreign firms to similar penalties for doing business with them.

Syria has emerged as a flash point for the global competition between the West and Iran as opposition forces seek to end the Assad family´s 40-year rule.

Mr. Assad is Tehran´s closest Arab ally. Syria and Iran have historically coordinated closely in arming and funding the militant groups Hezbollah in Lebanon and Hamas in the Palestinian territories.

U.S. officials hope that the overthrow of the Syrian regime will greatly diminish Iran´s ability to use Arab allies to target Israel.

U.S. and Arab officials said they first detected Iranian support for Mr. Assad´s crackdown last spring, largely in the form of surveillance equipment and intelligence-sharing. But these officials said they have also uncovered intelligence showing the Quds Force has been increasing its arms supplies to Mr. Assad´s security forces.

Many of the shipments are being delivered by the Qods Force using its fleet of Russian-made Ilyushin and Antonov planes flying over Turkish and Iraqi air space, according to U.S. officials.

The Treasury said that in March 2011 the Quds Force ordered a series of arms shipments into Syria using Yas Air´s cargo planes. The deliveries had been coordinated with Hezbollah and Syrian security forces, according to U.S. officials.

Turkey grounded one of the flights and found a large cache of weapons that had been listed on the flight´s manifest as "auto spare parts." Among the weapons were Kalashnikov AK-47 assault rifles, machines guns, nearly 8,000 rounds of ammunition and an assortment of mortar shells, the Treasury said.

Yas Air is based in Tehran. The U.S. and its European allies have been sanctioning Iranian transportation concerns, including shipping and port companies, in an attempt to cut off Tehran´s ability to ship arms and procure equipment for its nuclear program. (Copyright © Dow Jones & Company, Inc.) 03/28/12)