Ed Kassig, left, and wife Paula listen during a vigil for their son Abdul-Rahman Kassig. (Associated Press)
The parents of a US aid worker threatened with beheading by the Islamic State group say they broke their silence about their son's yearlong captivity because "the dynamics have changed" and they fear for his life.
Paula and Ed Kassig said during interviews broadcast on Monday that they've been doing all they can to free 26-year-old Abdul-Rahman Kassig since he was captured in Syria more than a year ago. That includes appealing directly to his captors via YouTube and Twitter messages.
A militant threatened Kassig's beheading in an October 3 video, in retaliation for US bombing attacks. The couple told "CBS This Morning" that the militants have issued demands that are beyond their power to meet.
"They demand. They simply demand," Ed Kassig said.
"We have sent them back messages that we cannot do what you ask. We have tried. But we don't have the power to do it," his wife added. They did not say exactly what the militants demanded, beyond the mention of US raids in the video.
Abdul-Rahman Kassig, who was born Peter Kassig but changed his name following his conversation to Islam while in captivity, was providing medical training and humanitarian aid to victims of the Syrian conflict when he was detained on October 1, 2013.
His parents said they were silent about his plight for a year at the instructions of the Islamic State militants but decided to go public after their son was named the terror group's next victim in a video that showed the execution of British hostage Alan Henning.
The video was the fourth released by the group. Previous victims were U.S. reporter James Foley, American-Israeli journalist Steven Sotloff and British aid worker David Haines.
"The dynamics have changed now. Steven's family kept to secrecy, and he was executed. Peter's name has been listed," Ed Kassig said.
Paula Kassig told NBC's "Today" show that the couple received an audio recording of their son a couple of weeks ago in which he said he feared his time was running out.
The couple on Monday also released additional excerpts from a letter written by their son and delivered by a former hostage.
In it, he said he was underweight but had no physical injuries and was "holding his own" but feared his time was short.
"I hope that this all has a happy ending but it may very well be coming down to the wire here," he wrote.