(CNN) A small number of ISIS detainees have "recently" escaped from detention by US-backed Syrian Democratic Forces, according to a US official with direct knowledge of the program. The official described the incident as involving "less than five ISIS fighters."

The fact that escape is possible has underscored the security worries: There have been urgent and growing concerns inside the US military and intelligence community about the security arrangements for hundreds of foreign ISIS fighters being held by the Syrian Democratic Forces, according to several US and coalition officials.

The fate of these fighters has taken on urgency because of the sheer number of ISIS operatives being held. US officials have said there are thousands of fighters in Syrian Democratic Forces hands, the majority of them Syrian nationals. Of the detained fighters, some 500 are foreigners from 40 countries, according to one of the officials.

As more fighters are captured and the detainee population swells, officials worry that the camps are becoming new ground for ISIS to regroup and develop new networks. The official with direct knowledge pointed out that in the days of large US-held detention facilities in Iraq, such as Camp Bucca, the US failed to realize quickly enough that such networks would take shape inside detainee populations.

ISIS leader Abu Bakr al-Baghdadi was held at Camp Bucca in 2004, and in retrospect many experts believe the beginning of ISIS could be traced to those days.

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