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New details on the 's former detention centres for foreign hostages in and on grave sites offer clues on where victims may be buried, a rights group said Thursday.

The US-based Syrian Justice and Accountability Centre (SJAC) said it had managed to pinpoint the exact locations of seven detention facilities once run by the extremist group.

It said it had identified three potential burial grounds after monitoring US court proceedings against a member of the notorious IS kidnap-and-murder cell dubbed "the Beatles".

The four-member cell, named after the pop band by their captives because of their British accents, was allegedly involved in the abductions of at least 27 people in Syria from 2012 to 2015.

The hostages, some of whom were released after their governments paid ransoms, were from at least 15 countries, including the United States, Denmark, France, Japan, Norway and Spain.

The IS tortured and killed their victims, including by beheading, and released videos of the murders for propaganda purposes.

Analysis: Eight years since its rise to power, IS is a shadow of its former self.

But ongoing conflict and instability in its former heartland are threatening to create conditions for the extremist group to make a resurgence

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The rights group said that, to corroborate information and trace hostage movements, it conducted further interviews with former hostages, victims' relatives and a second who pleaded guilty in the United States.

Information collected during the proceedings "provided evidence regarding the possible resting sites for both foreign and Syrian hostages known to be executed," the report said.

"Executions and burials often occurred at some distance from detention centers," in some cases less than a mile away, it added.

Two potential grave sites are believed to be located south of Raqa city, once the IS group's de facto Syria capital, SJAC researcher Gabriel Young said.

A third potential burial spot was identified in , now Syria's last major opposition bastion, he told AFP.

The rights group said it would conduct further investigations into the sites to confirm whether they contain corpses.

In March, a US trial opened against "Beatles" member El Shafee Elsheikh, who is accused of involvement in the murders of American journalists James Foley and Steven Sotloff and relief workers Peter Kassig and Kayla Mueller.

Another former cell member, Alexanda Amon Kotey, pleaded guilty in a US court in September 2021 and is facing life in prison.