With only 90 of the more than 5700 people in custody considered of foreign origin, the admission suggests the Bush administration may have exaggerated the role of outside fighters in stoking the Iraqi resistance.
The Bush administration has insisted all along that foreign fighters are playing a key role in Iraq, led in part by the Jordanian-born Abu Musab al-Zarqawi.
Of the foreign captives being held in Iraq, about half are from Syria; others are from Arab countries including Egypt, Iran, Jordan, Yemen and Saudi Arabia, US defence officials said.
The Bush administration has repeatedly accused both Syria and Iran of giving support to the resistance to occupation by making it easy for foreign fighters to cross their borders with Iraq.
But despite the small number of foreign fighters in custody, one senior defence official insisted that foreigners were involved in organising and financing attacks against US-led forces in Iraq.
"I think these people give backbone, a ruthless drive, to the insurgency," he said. "They recruit. They organise. They finance."
But analyst Anthony Cordesman of the Centre for Strategic and International Studies in Washington said: "The overwhelming mass of those involved in the insurgency are Iraqi nationals who are simply opposed to the US invasion and foreign occupation."