The Quds Force is a special paramilitary unit attached to Irans Revolutionary Guards whose mission is to help organise, train and equip militia in the Middle East.
The feared group, which enjoys a degree of autonomy from the rest of the Iranian military, is believed to have been created during the Iran-Iraq war in the eighties, when it helped organise the Kurdish insurgency in Iraq to fight Saddam Husseins forces.
It also played a role in the Soviet war with Afghanistan helping the Northern Alliance, which later joined US forces to overthrow the Taliban.
The US Department of Defence believes Quds Force members are supporting militias in Iraq, most notably by supplying insurgents with deadly armour-piercing roadside bombs known as explosively formed penetrators.
In July, US General Kevin Bergner accused Quds Force of being directly responsible for an attack in January in the Shia holy city of Karbala which killed five US soldiers.
It was the first time a US official had accused Quds Force over a specific incident, despite the belief in Washington that the unit has long been active in Iraq.
On Sept 20, US forces arrested an Iranian man during a raid on a hotel in northern Iraq and accused him of being a Quds member involved in smuggling roadside bombs and training foreign fighters in Iraq.
But Iraq criticised the arrest, saying he was a civil servant on an official Iranian trade mission.