The official, who spoke on condition of anonymity, said the Shiite "special groups" were being trained in Qom, Tehran, Mashad and Ahvaz in assassination and bombing techniques to target specific Iraqis as well as US troops and Iraqi security forces.
"We have intelligence reports confirming Iranian-sponsored groups are planning to return back to Iraq and are targeting specific coalition forces, ISF (Iraqi Security Forces) and Iraqi citizens," the official said.
The intelligence, if it proves out, raises the prospect of a deadly new security challenge at a time when the US military is hoping to make further cuts in its forces.
The official, who spoke from Iraq, said the information has been turned over to the Iraqi government "and they are taking the lead in handling the situation."
The groups were being trained in "reconnaissance, small arms, small unit tactics, cellular operations, EFPs and other IEDs, RPGs and assassination techniques," the official said.
EFPs, which stands for explosively formed projectiles, are armor-piercing bombs that have proven highly effective against US armored vehicles. The US military charges that components for the bombs are made in Iran.
The official said the special groups were being deployed to carry out "terrorist acts" against specific individuals as well as US and Iraqi forces.
The special groups have been associated in the past with radical cleric Moqtada al-Sadr's Mahdi army, but the official would not link those being trained to Sadr.
Among the Iraqi groups identified as involved in the training were Kitaib Hezbollah, which he described as a criminal group supported by the Iranian Revolutionary Guard that has claimed a number of sophisticated attacks since 2005.
The official identified a second Iraqi group as As Said Al-Haq.
"They are being trained by Quds Force under the leadership of Qassim Suleimani and Lebanese Hezbollah," the official said.
The US military many times in the past have accused Iran of fomenting violence in neighboring Iraq, supplying Shiite groups with arms and training for attacks on US forces.
But the violence has fallen off sharply in the wake of a US surge strategy that helped turn Sunni tribes against Al-Qaeda and Iraq's Shiite led government against Shiite militias and the so-called special groups.
US military officials have said many special group leaders fled to Iran, but were believed to be biding their time for a return.
Also contributing to the drop in violence has been a unilateral cease-fire declared a year ago by al-Sadr, who the US military believes is in Iran.
Sadr announced in June that he would replace the 60,000-strong Mahdi Army with a smaller fighting force to target the US-led occupation.
