Peter Kassig
Isis has threatened to kill US hostage Peter Kassig.Source: Handout

The family of Islamic State hostage, Peter Kassig, have released additional extracts from their son's letter to them whilst he is being held in captivity.

In the new letters, Kassig, seems calm about his situation, reassuring his family that "all in all I am alright."

The American hostage who is threatened with beheading, wrote: "I want to know about things here and what I've been through straight from me so you don't have to wonder, guess, or imagine (often this is worse than the reality). All in all I am alright. Physically I am pretty underweight but I'm not starved, & I have no physical injuries, I'm a tough kid and still young so that helps."

It is still really hard to believe all of this is really happening... as I am sure you know by now, things have been getting pretty intense. We have been held together, us foreigners... and now about half the people have gone home... I hope that this all has a happy ending but it may very well be coming down to the wire here, and if in fact that is the case then I figured it was time to say a few things that need saying before have to go.

Kassig, who converted to Islam and and is now known as Abdul Rahman, opens up about the difficulty of being held hostage by Islamic State, describing it as mentally "the hardest thing a man can go through, the stress and fear are incredible."

The 26-year-old humanitarian worker reveals both his current efforts to remain positive before contrasting it with initial struggle to understand his abduction.

"I am coping as best I can. I am not alone. I have friends, we laugh, we play chess, we play trivia to stay sharp, and we share stories and dreams of home and loved ones. I can be hard to deal with, you know me. My mind is quick and my patience thinner than most. But all in all I am holding my own."

"I cried a lot in the first few months but a little less now I worry a lot about you and mom and my friends."

He also appears resolute in the face of his likely fate: "They tell us you have abandoned us and/or don't care but of course we know you are doing everything you can and more. 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 & the stars.

Kassig, a former US solider, was captured on 1 October 2013. He had been travelling in an ambulance, heading to the eastern Syrian city of Deir Ezzour, where he was going to deliver essential medical supplies.

Kassig's parents have previously appeared in a video statement, pleading for their son's freedom.