After Peter Kassig's execution, the Islamic State group is left with one U.S. hostage, a 26-year-old female aid worker, The Daily Beast reported Monday, citing U.S. officials. The woman was reportedly captured last year in Syria while working with children displaced during the country's civil war.

The American woman’s name was not released as U.S. officials and her family reportedly fear that it could draw further attention to her and risk her life. She was reportedly captured along with several other aid workers, who were later released by the group, The Daily Beast reported, citing sources. Authorities told the publication that the woman was not featured in Sunday’s video, which is being seen as a recruitment effort by ISIS, and also shows the execution of several Syrian civilians, allegedly loyal to President Bashar Assad.

ISIS has demanded nearly $6 million in ransom for the release of the unnamed American woman, The Daily Beast reported, adding that the high ransom number signified that the group was aware its demand would not be met by the authorities, and also that the group had no intention of releasing its hostage.

According to Steven Radil, a professor of geopolitics and political geography at the University of Idaho, the beheading videos by ISIS are generally not aimed at U.S. audiences, but instead are geared to recruit foreigners eager to join the group in Syria and Iraq.

"I suspect that these videos are meant to serve a different purpose and communicate with a different audience," he reportedly said. "If I'm correct, these videos and other similar types of intimate, personal violence will be with us for some time to come."

The latest 15-minute long video is different from the previous beheading videos released by ISIS in that it does not specify who the group would target next for execution, raising further concerns about the militant group’s intentions.

A former U.S. counterterrorism official told The Daily Beast that the group would first consider public reaction before deciding on the fate of the remaining U.S. hostage. “Before they’re doing anything, they want to have a really good feel for how it will play,” the former official said, according to The Daily Beast.

The Islamic State group has previously killed several women and children, but has never shown the slaying of a Western woman on camera. Sunday’s video did not end with the Islamic State group’s pattern of introducing its next victim, but instead encouraged its followers to join their fight against the West.

Kassig was first shown in early October at the end of a video showing the beheading of British aid worker Alan Henning. And, in previous videos -- showing the beheadings of James Foley and David Haines -- the hostages were seen kneeling down and speaking in favor of the Islamic State group.

The latest video has yet again sparked global outrage against ISIS' actions, with President Barack Obama calling the beheading “an act of pure evil.”

British Prime Minister David Cameron tweeted: "I'm horrified by the cold blooded murder of Abdul-Rahman Kassig. ISIL have again shown their depravity. My thoughts are with his family."