Call on the Russian-supported (and sometimes Russian-staffed) secessionists to surrender. But rewarding the secessionists for killing 298 civilians (even in error as seems likely) by granting a ceasefire is grotesque.
Ukraine should argue that defeating the secessionists to regain control of eastern Ukraine is the best response.
Honestly, we've already rewarded Syria's use of chemical weapons by turning Assad into our disarmament partner (and some want to be his anti-ISIL partner, now).
Are we to make the Ukraine secessionists and their Russian masters our partners as a reward for this tragedy?