Up to 3,000 foreign insurgents may be fighting in Iraq, but they remain a small part of the overall rebellion, a US military analyst has suggested.
Iraq's insurgency boasts thousands of committed fighters
Algerians, Syrians and Yemenis are most numerous among foreign insurgents, said ex-White House aide Anthony Cordesman.
Mr Cordesman, a veteran analyst, used Saudi and other regional security studies to collate data on insurgents.
The figure is three times as large as unofficial Pentagon estimates, but may total no more than 10% of insurgents.
The Iraqi insurgency remains largely home-grown, Mr Cordesman added, with 90% or more hailing from Iraq.
"If there are anything like 3,000 fighters in Iraq, this poses a serious threat," Mr Cordesman said in a report for the Center for Strategic and International Studies, a Washington think-tank.
"In any case, the exact numbers are largely irrelevant. All it takes are enough volunteers to continue to support suicide attacks and violent bombing."
Most volunteers seep into Iraq through the porous border with Syria, the report says.
Others enter from Saudi Arabia, although increased vigilance has seen higher numbers prevented from entering Iraq.
Some 352 Saudis, or 12% of the total, were thought to have crossed into Iraq by August 2005, according to Saudi security services.
Jordan-born Abu Musab al-Zarqawi is Iraq's insurgency totem
There were also significant number of Sudanese and Egyptian militants.
Saudi security services reported that most Saudi fighters were aged 17-25, younger than those from elsewhere, who tended to be in their late 20s or early 30s.
But US and Iraqi officials in Iraq stressed the home-grown nature of the insurgency.
At the start of November 2005, only about 400 of almost 13,900 detainees in Iraq were foreigners, the report says.
"The foreign fighters' attacks tend to be more spectacular, [but] the local national, the Saddamists, the Iraqi rejectionists, are much more problematic," said Maj Gen Joseph Taluto, commander of a US infantry division in Tikrit.