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Iran's secretive Quds Force, accused by the United States of arming Iraqi militants with deadly bomb-making material, has built up an extensive network in the war-torn country, recruiting Iraqis and supporting not only Shiite militias but also Shiites allied with Washington, experts say.

Still unclear, however, is how closely Iran's top leadership is directing the Quds Force's operations — and whether Iran has intended for its help to Shiite militias to be turned against U.S. forces.

Iran likely does not want a direct confrontation with American troops in Iraq but is backing militiamen to ensure Shiites win any future civil war with Iraqi Sunnis after the Americans leave, several experts said Thursday.

The Quds Force's role underlines how deeply enmeshed Iran is in its neighbor — and how the U.S. could face resistance even from its allies in Iraq if it tries to uproot Iran's influence in Iraq.

The Quds Force — the name means "Jerusalem" in Farsi and Arabic — is the most elite and covert of Iran's military branches. Over the past two decades, the corps is believed to have helped arm and train the Hezbollah guerrilla group in Lebanon, Islamic fighters in Bosnia and Afghanistan, and even Sudanese troops fighting in south Sudan.

The force is part of Iran's Revolutionary Guards, which are separate from the regular military, report directly to Iran's Supreme Leader, Ayatollah Ali Khamenei, and are tasked with protecting Iran's Islamic government. The Quds Force, first formed in the 1980s and picked from the very best of the Guards, is its special branch for operations outside Iran.

"What Quds does is very specialized, the most dangerous work, operating underground," said Mahan Abedin, an Iran expert and the research director at the London-based Center for the Study of Terrorism.

Now the Bush administration is accusing the force of stirring up turmoil in Iraq.

Its key piece of evidence: "explosively formed projectiles," sophisticated roadside bombs that fire a slug of molten metal that can penetrate armored vehicles. The U.S. military says the Quds Force provided the materials to Iraqi Shiite militias, who used them to attack Americans.

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To make their case, U.S. military officials this week showed reporters in Baghdad pieces of EFP equipment, mortars and rocket-propelled grenades that they said were directly traceable to Iranian manufacture.

U.S. President George W. Bush told reporters Wednesday he could "say with certainty" that the Quds Force was providing the equipment to militants.

"What we don't know is whether or not the head leaders of Iran ordered the Quds Force to do what they did," Bush said.

Iran has denied the U.S. accusations. But the question of what the Quds Force and other Iranian operatives are doing in Iraq and how much Iran's top leadership directs that has become a key issue.

The Bush administration has increasingly blamed Iran for Iraq's chaos and taken a more confrontational stance, vowing to stop any intervention. That has raised worries among some Democrats in Washington that it is building a case for military action against Iran, a claim Bush denies.

The chief U.S. military spokesman in Baghdad, Maj. Gen. William Caldwell, said Iranian and Iraqi detainees in U.S. custody said in interrogations that "the Quds Force provides support to extremist groups here in Iraq both in the forms of money and in weaponry."

U.S. forces detained six Iranians in the northern city of Irbil in January, one of whom military officials say is the Quds Forces' operational commander in Iraq, Mohsin Chizari.

But the Quds Force help appears to go beyond militiamen attacking U.S. troops.

U.S. military officials have said it is supplying "rogue elements" of the Mahdi Army, a Shiite militia led by an anti-American cleric.

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But the Quds Force supplies training and some weapons to the Badr Brigade — a militia linked to the biggest Shiite political party — and smaller Shiite factions in the south, an official with a Shiite political party in Iraq who has close knowledge of militia activity told the Associated Press. He spoke on condition of anonymity because of the issue's sensitivity.

The Badr Brigade is linked to the Supreme Council for the Islamic Revolution in Iraq, which is headed by cleric Abdul-Aziz al-Hakim, one of Iraq's most powerful politicians, who met with Bush at the White House in December.

America's Kurdish allies also have past links with the Quds Force, which helped them against Saddam Hussein's regime in the 1990s. Notably, the six Iranians seized by U.S. troops were in Kurdish-controlled Irbil.

In addition to supplying weapons to Iraqi militias, the Quds Force has been recruiting Iraqi Shiites, giving them up to $150 a month and sending some to Iran for training, the Shiite political party official told AP.

Caldwell acknowledged that detainees had said, under interrogation, that Quds operatives were supplying weapons to factions in the Iraqi government. He said U.S. officials had asked political parties and government officials about the material.

"Some explained that there is a need for certain weaponry to come in for protection purposes," he told a Baghdad press conference Wednesday. "The concern we had ... is that on that list were sniper rifles, mortars and some elements that are clearly offensive in nature."

At most, Iran's entire Quds Force probably only numbers about 2,000 — only around 800 of them core operatives, Abedin said.

He doubted the Quds Force was directly giving militias weapons but said they undoubtedly were providing intelligence and other organizational help.

"It would be very incriminating and dangerous for Iran to directly supply weapons to the militias, and its not a part of Iranian policy to directly confront the Americans," he said.

Instead, the goal is likely "to enable these armed formations ... to gain an advantage over their Sunni rivals" in the battle for power that Iran expects could erupt later.

"They are looking to beyond, when the Americans withdraw," he said. "They see the Shiite militias as natural allies."