I honestly don't see much point in trying to analyse this until waverider can clarify the intended meaning. My feeling is that the sentence originally arose in waverider's mind as an expression of this thought:
The armed forces, seeking to take control of the territory, were brutal.
If that's the case, I'll argue that there is no semantic equivalent to a relative clause (whether that be who were seeking or who sought). My interpretation is that the gerund-participial clause is logically connected to the sentence meaning in that the armed forces' brutality was a result of their seeking to take control. You could head by at the beginning of the phrase to show this more clearly, and neither who were seeking or who sought are possible.
Of course, I may be misinterpreting the sentence. My point, though, is that if you want to syntactically analyse any real language in use properly, you ought to get the semantics nailed down first.