The parents of an American hostage threatened with beheading by Islamic State militants said in an interview with "CBS This Morning" on Monday that they are unable to meet the financial and other demands of their son's captors.The parents of Abdul-Rahman Kassig, an aid worker abducted on Oct. 1, 2013, did not elaborate on what Islamic State had demanded in exchange for their son's freedom."Their demands have always been ones that we cannot accommodate," Paula Kassig, the captive's mother, told the TV program."CBS this Morning" reported that Kassig's parents received an audio recording of their son two weeks ago in which he warned that he was in danger if the United States did not halt its air strikes.The 26-year-old captive, an Indiana native, was threatened in a video issued earlier this month by Islamic State that showed the beheading of British aid worker Alan Henning, 47.Kassig's parents told the TV program that they had kept his capture secret for roughly a year on orders from Islamic State militants but that the recent beheading of US journalist Steven Sotloff after his family had followed similar instructions moved them to change tactics.Kassig's family on Monday also made public additional portions of a letter from their son they received earlier this year from a hostage released by Islamic State.In it, Kassig said he is kept together with other hostages, with whom he plays chess and trivia games, but that the mental strain of captivity has been "incredible.""Don't worry Dad, if I do go down, I won't go thinking anything but what I know to be true. That you and mom love me more than the moon and the stars," the letter said.A former US Army soldier who deployed to Iraq in 2007, Kassig was doing humanitarian work through Special Emergency Response and Assistance, an organization he founded, when he was taken captive while on his way to the eastern Syrian city of Deir al-Zor, his family has said.Kassig's first name was Peter before he converted to Islam while in captivity, the family has said.Henning's beheading was the fourth such killing of a Westerner by Islamic State, following the deaths of two US journalists and another British aid worker.