Iran's export of so-called manpads -- antiaircraft missiles that can be carried by a single person -- represent what Panetta called a dangerous escalation.
"There is no question when you start passing manpads around, that becomes a threat—not just to military aircraft but to civilian aircraft," Panetta told The Wall Street Journal in an interview describing shifting threats to the U.S. as he prepares to leave his post. "That is an escalation."
Western officials have long worried about the spread of such weapons and the risk they pose to airline passengers as well as to military helicopters and jets.
Recent U.S. intelligence pointed to new efforts by Iran to smuggle manpads, but few shipments had been intercepted before Jan. 23, when Yemen, aided by the U.S., intercepted a boat carrying the weapons.
"It is one of the first times we have seen it," Panetta said.
U.S. investigators said evidence indicated the missiles were supplied by the Islamic Revolutionary Guard Corps, Tehran's paramilitary force.
Iranian officials didn't respond to requests for comment about distributing weapons to regional allies.
Mr. Panetta said the U.S. is stepping up efforts to counter the Iranian threat, and is leading a multination exercise in the United Arab Emirates though Feb. 7 to improve the interdiction of Iranian arms and other weapons. The defense secretary called the exercise critical to building up Arab capabilities to help halt Iranian arms transfers, including the smuggling of manpads.
The disclosures by Mr. Panetta came as he prepares to step down after 19 months as defense secretary, a period marked by an intensified focus on Iran as concerns mount about its nuclear ambitions, an expanded campaign of drone strikes against militants in several countries, and the emergence of a new Al Qaeda haven in Africa.
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