James Jeffrey, the US Special Representative for Syria and global head of the coalition to defeat ISIS, said Wednesday that more than 100 ISIS fighters had escaped detention since Turkey invaded northeast Syria."We would say that the number is now over 100. We do not know where they are," Jeffrey said when asked if the US knew where the fighters were or if the US could monitor them without US forces present.Visit Business Insider's home page for more stories.James Jeffrey, the US Special Representative for Syria engagement and global head of the coalition to defeat ISIS, told Congress on Wednesday that about 100 ISIS prisoners have escaped detention since the Turkish invasion of northeast Syria and that the US does not know their whereabouts.During a meeting with the House Foreign Affairs Committee on Wednesday, Jeffrey told New York Democratic Rep. Eliot Engel that over 100 ISIS prisoners had so far escaped the custody of the Syrian Democratic Forces, reinforcing Secretary of Defense Mark T. Esper's statements about escaped ISIS prisoners on Tuesday"We know from previous briefings these ISIS detainees are among the most dangerous fighters, intent on attacking the United States and its allies," Engel said during Wednesday's hearing. "Does the US have an idea where these individuals are, and is the US able to monitor or effectively operate against ISIS, given the withdrawal of US forces'""We would say that the number is now over 100. We do not know where they are," Jeffrey answered.Experts have previously told Insider about the risk of ISIS fighters breaking out of prison amid the chaos of the Turkish incursion into northeast Syria."We know in the past, they've done prison breakouts. They've seen it as a top priority," said Daniel Byman, a senior fellow at the Brookings Institution's Center for Middle East Policy.During a press conference on Friday, President Donald Trump insisted the situation in northeast Syria was under control.ISIS fighters are "under very, very strict lock and key," Trump said in the Oval Office. He did concede that "a small number got out, relatively speaking," but, contradicting Jeffrey, he said "they have largely been recaptured."It's unclear what role US forces will play in fighting ISIS in Iraq and Syria going forward. The soldiers retreating from northeast Syria were to be redeployed to Iraq, whose government said the troops could only transit the country and would not be allowed to redeploy there. Trump has said that a small number of soldiers will remain in Syria.Insider reached out to the State Department for further comment on Jeffrey's remarks but did not receive a response by press time.NOW READ:US officials are considering pulling nuclear weapons from Turkey, effectively severing the US-Turkey allianceSEE ALSO:Trump claimed the US captured all ISIS fighters, but they just launched a major attack on an Iraqi oil fieldJoin the conversation about this storyNOW WATCH: What the US needs to do to prevent a new ISIS in Iraq Click here to read full news..