A British delegation, including an imam from London, have visited Sudan to try to dissuade young British doctors from joining Islamic State (Isis), which has been urgently seeking more foreign medics to help at its hospitals in Syria.

The Foreign Office is coordinating efforts to prevent more Britons travelling from Khartoum’s University of Medical Sciences and Technology (UMST). At least 17 British doctors travelled from there to Syria during 2015 to staff Isis’s health ministry.

It has emerged that a second group of UK doctors who left Sudan for Syria have joined up with members of an earlier group who travelled to join Isis in March. According to family sources, the second group of five Britons, including two brothers from Leicester, are understood to have joined up with 20-year-old Rowan Kamal Zine El Abidine, one of a group of nine British medical staff who journeyed from Khartoum months earlier.

Both groups were said, initially at least, to be under the control of Mohammed Fakhri, a 25-year-old from Middlesbrough who also attended UMST and styles himself as the recruiter of British medics for Isis. Currently believed to be in Raqqa after evading attempts to capture him in Turkey, Fakhri ran UMST’s Islamic Cultural Association, where he radicalised his fellow British students. The association has been shut down and numerous online traces of the group have been removed following pressure from the UK authorities.

Fakhri himself appears to have gone quiet, his last known act being to publish an essay three months ago titled “To the Hesitant”, in which he attempts to reassure those who are having reservations about his project. In an earlier essay Fakhri conceded that the caliphate’s medical expertise lags behind its western counterparts, admitting: “The Islamic State realises that in fields like medicine, the non-Muslims are currently those advanced.”

Meanwhile, family sources of the young medics have told the Observer they remain in touch with their siblings and children inside the self-styled caliphate. In fact, some say contact has improved over recent months.

“Communication has become less controlled than it was. At one point we were worried they may have had their phones confiscated by Fakhri,” said a sibling of one of the initial nine medics inside Syria, who agreed to speak on condition of anonymity.

All, however, appear to have been ordered not to disclose their whereabouts, although it appears from sporadic sightings and online postings that the doctors are currently spread across the caliphate. Some appear to be stationed at a specially built medical faculty next to the Raqqa National hospital; others have been dispatched to the city of Al-Bab, east of Aleppo, and are based at Menbij’s National hospital, 12 miles from the Turkish border, or have been sent towards Iraq and the city of Deir ez-Zor, deep inside Isis territory.

Despite Isis’s relative success in luring young foreign doctors to Syria – Australian, Russian, Libyan and Saudi doctors have all travelled to join the militants – the group is urgently requesting more.

At least one British doctor has posted links to Tumblr accounts – now deleted – which admit that more doctors are required to join the dawlah or the state, but that it is getting harder to enter the caliphate. The account of Abu Sa’eed al-Britani said that crossing the mountains between Turkey and al-Sham (Syria) is fraught with difficulty. “I don’t believe it’s a feesible [sic] option now unless you are 100% certain that you will enter dawlah territory and do not mind the negative consequences of getting caught,” he wrote. Instead he advises potential medical recruits to consider joining Libyan, Boko Haram or Isis affiliates in Egypt.

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

A Foreign Office source confirmed that its embassy in Khartoum was working with the university and had recently sent over a delegation headed by imam Luqman Ali, director of the Khayaal Theatre Company in London, which adapts Muslim literature not previously performed on stage.

Whitehall officials shared counter-extremist advice with lecturers and students based on work in UK universities under the Prevent programme, the government’s counter-terrorism initiative, which has been criticised for discriminating against Muslims.

An FCO spokesman said: “Since the cases of British nationals travelling from Sudan to Syria, the British embassy in Khartoum remains in contact with the University of Medical Sciences and Technology to address these concerns. We continue to support the university in its efforts to promote dialogue around these issues.”

Meanwhile, Facebook messages leaked to the Observer reveal the speed with which Fakhri was himself radicalised while a student in Khartoum. They show that in August 2014 he was still talking enthusiastically about his “postgraduate job” and pathology exams, yet within months he is thought to have travelled to Syria before persuading others to follow him.

They also reveal how, when Fakhri took over as president of the Khartoum university-based Islamic Cultural Association, he planned to bring extremist preachers on to the campus and altered the organisation’s structure. In August 2014 he divided members into classifications along the lines of male and female, religious and unreligious, Muslim and non-Muslim.

One theory is that one of Fakhri’s roles for Isis was to recruit female medics, who are prohibited under Isis from operating on males. Seven female British medics, including Lena Mamoun Abdelgadir, a 19-year-old medical student from Norfolk, are among those who have joined Isis in from Khartoum.