The primary threat to the Americans comes from three Shiite militia groups operating in Iraq, which officials said they believe are being trained and equipped by Iranian Revolutionary Guard Corps special forces, Major Genereal Jeffrey S. Buchanan, chief spokesman for the U.S. military in Iraq said, The Washington Post reported.
"All of them receive at least indirect support from elements in Iran," Buchanan said in an interview this week.
In early June, what U.S. officials believe was a sophisticated rocket slammed into a joint Iraqi-U.S. military base in eastern Baghdad, killing six American soldiers in the deadliest single attack on forces here in more than two years. In addition, three U.S. troops were killed by roadside bombs in June.
Last week, an American contractor for the U.S. Agency for International Development was killed when suspected Shiite militants attached a bomb to a car he was riding in near a Baghdad university. And Sunday, two U.S. troops were killed when an apparent armor-piercing grenade was lobbed at their vehicle.
Although the U.S. military did not release specifics on Wednesday's attack pending notification of next of kin, officials familiar with the incident said the rocket was so powerful that it also wounded more than a dozen soldiers, several critically.
There was no immediate assertion of responsibility for that attack. However, Kataib Hezbollah, one of the Shiite militia groups mentioned by Buchanan, said earlier last month that it was responsible for the attack that killed six soldiers.
Buchanan said there is "no doubt" that Kataib Hezbollah "follows orders" from the Iranian Revolutionary Guard Corps's Quds Force, a highly specialized unit responsible for operations outside Iran. "Their leadership lives in Iran, they are directly trained by the Quds Force and they are supplied by them," Buchanan said.
Buchanan said efforts to protect U.S. forces in Iraq are further complicated by rival Shiite militias that are vying to emerge as the dominant Shiite insurgency group in Iraq.
In an interview with Bloomberg News, outgoing Defense Secretary Robert M. Gates expressed similar frustration with Iran's ties to the Shiite militias operating in Iraq. Iran is "facilitating weapons, they're facilitating training, there's new technology that they are providing," Gates said. "They're stepping this up, and it's a concern."