A few weeks ago Ukrainian military began to build up its military near Donbass self-proclaimed republics - Donetsk People's Republic and the Lugansk People Republic.
There are talks of launching an offensive on Donbass.
Russia, begun to mirror the movement, moving its own forces closer to the Donbass republics. Being the guarantor of the Minsk agreement they have the obligation to protect the said republics. In 2015, Minsk agreement stopped the Donbass militia advance but guaranteed that the conflict will be resolved by political means.
Ukraine at the time was at a disadvantage, had no problem signing the agreement, but now that they re-grouped and feel like have the military advantage over the Donbass militia, they do not feel like abiding by the agreement any longer.
This is how the US State Department reacts: