The State Department commended the government in Yemen after it said it seized a cache of Iranian-supplied man-portable anti-aircraft missiles and rocket-propelled grenades.
"These weapons are clearly designed to cause significant damage with the highest possible number of casualties and are a threat to both Yemen and the region," department spokeswoman Victoria Nuland said in a statement.
The Shiite government in Iran is defying international expectations with its proliferation activities, she said.
Last week, Nuland praised Yemen for moving forward with plans for a national dialogue that's said to include southern separatists and backers of the rebel Houthi movement, a northern Shiite group.
Iran's state-funded broadcaster Press TV reports the allegations come as the United States increases its controversial drone program over Yemeni territory. A missile fired from an drone in 2011 killed Anwar al-Awlaki, a U.S. citizen who was the ideological leader of the Yemeni branch of al-Qaida.