Overall, US officials reported that the number of foreign fighters entering Iraq this year dropped from 80-110 a month in the first half of the year to around 40 in October, partly due to the Sinjar raid.

After the raid the number of suicide bombings in Iraq fell to 16 in October - half the number seen during the summer months and down from a peak of 59 in March. US military officials believe that 90% of such bombings are by foreigners.

The captured data has been described as an intelligence treasure trove that included biographical details and the hometowns of the more than 700 fighters who entered Iraq since August 2006. Of those 307, or 41%, were Saudis and 137, or 18%, Libyans, senior US military sources told the New York Times.

Saudi Arabia, former home of Osama bin Laden and 15 of the 19 September 11 hijackers, has cracked down hard on al-Qaida in recent years. Saudi intelligence works closely with its US counterparts, but there have long been suspicions that the country's most dangerous jihadis have gone to Iraq. "The border with Iraq is much more carefully controlled than it was 18 months ago," said one British official. The Saudis also run extensive programmes "re-educating" and rehabilitating fighters who have returned from Iraq or Afghanistan to see "the error of their ways".

The US, Britain and others have praised the Saudis for their efforts, pointing especially to a recent appeal by the kingdom's grand mufti, Sheikh Abdulaziz al-Asheik, who condemned "mischievous parties" who send young Saudis abroad to carry out "heinous acts which have no association with Islam whatsoever".

The presence of a large number of Libyans among the insurgents in Iraq suggests that the regime of Muammar Gadafy has pushed its violent Islamist enemies abroad, having cleaned up its own act by renouncing support for terrorism and surrendering its chemical and nuclear arsenal after the US invasion of Iraq.

The documents, found in September, showed that the third-largest source of foreign fighters was Yemen, with 68, followed by Algeria, 64, Morocco, 50, Tunisia, 38, Jordan, 14, Turkey, six, and Egypt, two. These figures seem to corroborate suggestions of a worrying increase in jihadi activity across north Africa, where armed groups from Libya, Algeria, Tunisia and Morocco have united under the banner of al-Qaida in the Islamic Maghreb.