Alireza Jafarzadeh, a former spokesman for the opposition National Council of Resistance of Iran, said Tehran was training fighters with light weapons, mortars and the roadside bombs used to such deadly effect against US troops.
"We’re not talking about some rag-tag elements and individuals who go out of their way and happen to provide weapons or assistance to a number of Shiite militia groups," Jafarzadeh told reporters in New York.
"We are talking about systematic, well-organised, high-level training officially provided by the Iranian regime."
He said that militants who joined the camps — alleged to be near Tehran and further south near Jalilabad — were trained either as special forces, snipers or in the use of anti-aircraft missiles.
Jafarzadeh, who back in 2002 was among the first to claim that Iran was intent on developing a nuclear weapons program, said his information came from well-placed sources inside Iran who could not be identified.
He did not say how many people were believed to have passed through the alleged camps, but said the numbers were certainly in the thousands.
"This is the largest operation by any foreign entity in Iraq. That is not limited to only money, it includes explosives, arms, intelligence, all of that," he said.
He said his sources believed that the vast majority of the militants were Iraqis from the Shiite parts of Baghdad and the southern provinces of Najaf, Karbala and Basra.
He alleged that one of Iraq’s most powerful Shiite leaders, Abdel Aziz Hakim, leader of the Supreme Council for the Islamic Revolution in Iraq (SCIRI), whose party sits in the Iraqi government, was implicated.
"This operation involves a huge amount of coordination, high-level coordination at the highest level," he insisted.
Some of the militants allegedly entered Iran disguised either as pilgrims or as people requiring medical attention, while others either crossed the border illegally or with visas issued by Iranian officials in Iraq, he said.
Jafarzadeh stepped down as spokesman for the National Council of Resistance of Iran in 2003. The group is considered a front for the People’s Mujahedeen Organisation of Iran, which the United States classes as a terrorist group.