The head of the Sudanese university where more than 20 young Britons were recruited by Islamic State has claimed that the group’s recruitment machine has been eradicated from the campus.

Attempting to reassure British parents that it is safe to send their children to Khartoum’s University of Medical Sciences and Technology (UMST), Dr Ahmed Babiker said that counter-radicalisation efforts alongside the dissolution of a controversial faith group had successfully extinguished Isis from the site.

Babiker said: “I am happy that it is over, we can move on. It is almost one-and-a-half years since the last person went to Syria – we have closed down the operation.”

Last week, two British students from UMST, Ahmad Sami Kheder, 25, from London, and Hisham Fadlallah, 24, originally from Nottinghamshire, were killed in fighting in Iraq.

Kheder is understood to have been in a convoy attempting to leave the besieged city of Mosul when it was struck by gunfire, most likely from Iraqi forces who are advancing steadily towards Isis positions from the south.

On Saturday, details emerged of what might be the first chemical attack in Iraq’s second largest city after 12 people were treated for possible exposure to chemical weapons agents. All the victims have been transferred 50 miles to Erbil, the capital of Iraq’s Kurdish region, with four of them showing “severe signs associated with exposure to blister”.

Eighteen UK doctors who were studying at UMST before entering Syria to join Isis are thought to be still trapped inside the group’s shrinking caliphate. So far, at least four of the British medics are known to have been killed, with parents increasingly anxious that others will be caught up in fresh fighting or unable to safely flee territory controlled by Isis. Recent reports suggest Isis has kidnapped individuals attempting to escape while airstrikes continue to target extremists. Seven foreign fighters belonging to Isis were killed in an airstrike west of Mosul on Friday night.

Among the British doctors who studied in Sudan and joined Isis are Leicester brothers Mohamed and Ibrahim Ageed, aged 23 and 25, who both attended fee-paying Loughborough Grammar School and entered Syria in June 2015. Their father has always argued that his sons should not be considered terrorists despite joining Isis, stating that there is no evidence that they have ever volunteered to fight.

Adil Ageed told the Observer: “We as a family believe that my two sons Mohamed and Ibrahim have attempted to travel to Syria purely for humanitarian reasons with the intention to provide medical aid.”

It is understood that a number of parents remain in contact with their children inside the caliphate, mainly through WhatsApp. Pinpointing the whereabouts of the young British medics is among a number of objectives set by the British security services. In particular intelligence agencies are keen to identify what has happened to Mohammed Fakhri, 24, from Middlesbrough, who is believed to be the chief recruiter of the medics in Khartoum.

Fakhri, who is of Palestinian origin, invited radical preachers to events on the UMST campus and is believed to have travelled to Syria in 2015 before returning to Sudan and to have encouraged others to follow. He published an internet essay telling British Muslims they were “obliged” to defend Isis’s self-declared caliphate. A source at the Abu Bakr Mosque and cultural centre in Middlesbrough, where Fakhri’s father is a trustee, said on Saturday that Fakhri had not been seen for a long time. “I have not heard what’s happened to him, only that he’s in Syria fighting for the Syrian people but it’s something that we do not get involved in.”

One of the British Sudanese doctors, Rowan Kamal Zine El Abidine, 22, was the first British female recruit to be killed in an airstrike in Iraq last July. A year earlier, another of the British Sudanese medics, Osman Mustafa Fagiri, 23, who formerly lived in Chalk Farm, north London, was killed fighting Syrian government forces.

The number of UMST students who joined Isis is believed to have been as high as 40, with the first group of British students entering Syria in March 2015 and a subsequent group following their trail three months later, including the daughter of a senior Sudanese government official.

Concern that more sleeper cells of British doctors are biding their time in Khartoum before heading to Syria has reportedly prompted some parents to withdraw their children from UMST and triggered considerable unease among UK diplomats and the Sudanese intelligence services.

The university charges £1,500 a year for tuition compared with £9,000 in the UK, and its qualifications are recognised by British medical authorities.

In Sudan, intensive efforts have been made to disrupt Isis recruiting cells, with the ministry of interior in Khartoum announcing that about 70 Sudanese had gone to join Isis franchises in Libya and Syria. One UMST student, Tarik Hassane, was jailed in 2014 for a minimum of 21 years for plotting terror attacks, identifying Shepherd’s Bush police station in west London and the nearby Parachute Regiment Territorial Army barracks in White City as possible targets.

HOW THE STORY UNFOLDED

When, in March 2015, the Observer revealed that a group of British medics had entered Syria the embattled caliphate of Islamic State (Isis) was a very different place, busily recruiting doctors from across the world, supposedly as part of an emerging state infrastructure.

Although little is known about what then happened to the group, all of whom studied in Sudan, sources in Syria have revealed they initially attended mandatory sharia camps for up to 15 days, surrendered their passports, and were subsequently taken to the group’s de facto capital, Raqqa.

Weeks earlier, in January 2015, Isis had celebrated the opening of a training facility for young foreign medics as part of its heavily promoted Diwan al-Siha (health ministry). There, in a building opposite Raqqa’s National Hospital, the young medics were trained by Isis surgeons. Lessons were in English, with women and men taught separately due to the group’s strict segregation policy. By May 2015, most had been dispersed throughout Syria. Sources said that three are believed to have been sent to Isis hospitals in Jarabulus, two to al-Bab and two to Manbij, with the rest remaining in Raqqa.

Now Jarabulus has been liberated from Isis by Turkish forces, the former stronghold of al-Bab has been wrested from the group by Turkish and rebel fighters, Raqqa is increasingly encircled by a Kurdish-led coalition and Manbij has fallen. Raqqa’s main hospital has also been damaged in coalition airstrikes.

Increasingly, the question is whether as many as 18 young doctors who attended the University of Medical Sciences and Technology (UMST), and then joined Isis, can escape the Middle East alive.