(中尉は)両手で刃{やいば}を腹{はら}の奥深く押さえつけながら、引き廻して行かねばならぬのを知った。
My trouble lies with
刃を腹の奥深く押さえつける
If I consider 奥深く to act as adverbial form of 奥深い, then I don't know what is modified by 腹の: taking の 助詞 to work in attributive role modifies nouns, and there is no noun here; taking の to work as an agent case marker doesn't make sense to me, as there is nothing for 腹 to act upon.
Taking 奥 as a noun doesn't make sense either because it's missing a case marker, something like
刃を腹の奥に深く押さえつける (with に in allative role)
In other words, how does の work here?
A bit more if necessary for context:
中尉は右手でそのまま引き廻そうとしたが、刃先は腹{はらわた}にからまり、ともすると刀は柔らかい弾力で押し出されて来て、両手で刃を腹の奥深く押さえつけながら、引き廻して行かねばならぬのを知った。