The clause before the comma is a conditional clause. It looks strange because it does not begin with the conjunction IF.
The writer wants to say:
in case there is no consiliatory moves (= in case they are absent), some things will follow
We could add the missing IF, do away with absent, and reword it thus:
If some decisive and conciliatory moves are not taken by the rebel leadership in Donetsk, political pressure will shift to Moscow as the political context of the war shifts in two crucial ways.
If the rebel leadership in Donetsk fails to take some decisive and conciliatory moves, political pressure will shift to Moscow as the political context of the war shifts in two crucial ways.
Note that the word order in the conditional clause you cite is changed: not the standard
Consiliatory moves from the rebels are absent, ...
Absent (some) consiliatory moves, ...
I cannot say exactly why the verb are disappears with the changing of the word order. Maybe someone with more knowledge will shed light on this. I feel that if it stayed, the clause would not have been a conditional clause.