Teenage twin sisters from Manchester have assumed "influential roles" among a group of British women who have travelled to Islamic State (IS, or Isil) territory, according to a report.
Zahra and Salma Halane used social media accounts to contribute to the extremist group's propaganda and encourage other women to join them, experts claimed.
The 17-year-olds are believed to have travelled to Syria last June.
In Till Martyrdom Do Us Part: Gender And The ISIS Phenomenon, authors Erin Marie Saltman and Melanie Smith said they "form a particularly vocal sub-cluster of the British female migrant contingent".
They said: "Throughout many months of monitoring, it is apparent that the Halane twins have taken on influential roles within the cluster of British women who have journeyed to Isis territory."
Analysis of the girls' social media accounts indicate that they are "resilient" in overcoming online censorship by alternating usernames, changing Twitter handles and using "shout out tactics" to regain follower networks, the report said.
It went on: " This has allowed for their voices, like many others, to contribute persistently to the propaganda narratives perpetuated by female migrants to IS territory, bolstered by the knowledge that their accounts have established influence.
"Zahra and Salma have utilised various platforms to achieve such influence, predominantly Twitter and Instagram."
The analysis claimed that the sisters have "radicalised further" during their time in IS-controlled territory, particularly since the deaths of their jihadist husbands.
It said: " It is important to note that the content produced by both twins since the beginning of 2015 has become consistently and markedly more 'extreme'.
"Similarly, both Zahra and Salma seem to have placed more emphasis on encouraging Western women to make 'hijra' to the territory since the beginning of 2015, capitalising upon the media attention generated by their disappearance.
"Much of this encouragement is subtle, achieved by representing daily life in Isis territory in a positive light and by creating a vision of a utopian society in which their values are venerated and shared."
The report by the Institute for Strategic Dialogue and International Centre for the Study of Radicalisation and Political Violence at King's College London identified differences between the twins.
It said: "Salma appears to contemplate upon the evolution of her life in Manchester to her life in the Isis as a continuum, whereas Zahra appears to have separated these two episodes entirely - rarely making mention of personal details or referencing family."
Separate claims emerged today that three runaway British schoolgirls from east London who are feared to have joined militants in Syria are being trained for special missions.
Police revealed earlier this month that more than 700 British extremists have travelled to Syria.