They are searching for the bodies of those killed by Mohammed Emwazi, the masked murderer known as Jihdai John who was filmed beheading multiple Westerners in sickening Isis propaganda films.
CNN reported the site was revealed from two British ISIS fighters who were captured in eastern Syria last month.
Like Emwazi, the informants, Alexanda Kotey, 34, and El Shafee Elsheikh, 29, were part of the “Beatles” terror cell.
But identifying exactly where the grave is and the remains could prove difficult because of the sheer quantity of bodies spread across different locations.
A source said: “The two suspects have revealed clues about where the hostages are buried. But that may not be enough.
“The US wants to take the men back to the burial site areas so that they pinpoint the graves.”
The Beatles terror squad, so-called because of their accents, are thought to have murdered at least 20 people.
In January 2017, US authorities named Kotey as a member of the cell and said they had imposed sanctions on him.
He appeared in a number of vile videos in which captives including British aid workers Mr Haines and Mr Henning were beheaded.
A fourth member of the cell, Amine Davis, was convicted of being a member of a terrorist organisation and jailed for seven-and-a-half years at a court in Silivri, Turkey, in May 2017.