Bing blows it on the conjunction:
Russian has two words for "and" - и and a. This one, a, is a "contrastive and" (I like cats and he likes dogs), and "and" is rarely the best choice. "But" is better, "while" is my personal favorite for most contexts. Here, "but rather" is what you want.
Again, Google Translate does better: "I want to emphasize, we believe that this is not an incident, not a disaster but a terrorist act."
BUT they neither one do right by Poroshenko's verb считаем; it's really in the "consider, judge, count, reckon, esteem, look upon, hold to be" semantic field. Poroshenko isn't saying he believes it's an act of terror; he's saying he considers it one. Big important difference.