A spokesperson for the Syrian Democratic Forces (SDF) said they were among five men preparing a terror attack on civilians fleeing ongoing battles.
The SDF, a coalition of Kurdish-led militias fighting Isis with international support, has recently driven militants out of the city of Hajin and is closing in on al-Shaafah, where two British soldiers were injured on Saturday.
A spokesperson said: “A group of terrorists who had been preparing to attack the civilians who were trying to get out of the war zone in masses was detected.
“Following long-term technical and physical follow-up, an operation against the cell was carried out by our forces.
“As a result of the operation, five terrorists originally from the United States, Ireland and Pakistan were captured.”
The SDF identified the captives as Warren Christopher Clark, who went by the war name Abu Mohammad al-Ameriki, from Texas and fellow American Zaid Abed al-Hamid, 35, known as Abu Zaid al-Ameriki.
A man said to be from Dublin in Ireland was named as Alexandr Ruzmatovich Bekmirzaev, 45, and the two others detained were Fadel al-Rahman Jad, 48, and Abed al-Azem Rajhoud, 19, both from Pakistan.
A LinkedIn account in the name of Alexandr Bekmirzaev could not immediately be confirmed as belonging to the same person, but gave details of a man from Dublin of a similar age.
It said he was originally from Belarus and worked as a sales assistant at the city’s famed Arnotts department store from 2005 to 2007, and later as a security guard.
The page, which does not appear to have been updated for some time, calls Mr Bekmirzaev a “security and investigations professional”.
Clark was previously identified as an Isis recruit by the Program on Extremism at George Washington University.
In a report published last year, researchers included the 34-year-old’s application letter for a job teaching English at a university in Mosul.
The document, retrieved from a house used by Isis members before they were driven out of the Iraqi city, said: “I am looking to get a position teaching English to students in the Islamic State … I consider working at the University of Mosul to be a great way of continuing my career.”
Clark wrote that he wanted to create a “supportive learning environment” for students and that he had taught different subjects to pupils of all ages in the US.
“Please feel free to contact me ta your convenience, thank you for taking time to consider me,” concluded the letter, which was sent with a CV.
NBC News reported that Clark, of Sugar Land near Houston, had a bachelor’s degree in political silence and global business.
He reportedly converted to Islam around 2004 and was drawn into radical videos on the internet.
Clark left the US for Syria in 2014 but his eventual role in Isis remains unclear. The group has closed conventional schools and universities in its territories and conscripted boys as child soldiers.
The University of Mosul was shut down and looted by terrorists in 2014, before being reopened to teach an Isis-approved medical curriculum while being used as a military base and weapons factory.
Researchers said an estimated 300 Americans attempted to join Isis and other Islamist groups in Iraq and Syria, including a small number who rose to senior positions. Only a dozen are known to have returned to the US.
Ireland’s Gardai police force estimates that around 30 extremists left the country to join Isis.
Speaking at an Interpol conference last year, Assistant Commissioner for National Security and Intelligence, Michael O’Sullivan, said that “quite a number of those are deceased” and others are missing.
Numerous British Isis fighters, including alleged members of the “Beatles” terror cell that included executioner Jihadi John, are also being held by SDF.
The Kurdish-led alliance has called for militants’ home countries to repatriate them for trial, but the UK is among nations stripping terrorists of citizenship to prevent their return.
Donald Trump claimed Isis was “defeated” in Syria last month and ordered the withdrawal of US troops from the country, but no timetable has been set.
The move sparked global condemnation over fears that Turkey, which views the YPG as a terrorist group, would invade and that Isis could use the lack of airstrikes to regroup.
The Pentagon assessed there were still as many as 14,500 Isis fighters in Syria in August.
Mr Trump’s national security adviser, John Bolton, said on Saturday that the US military would only withdraw if the militants were defeated and Turkey assured the safety of Kurdish allies.