Usage of ''as many as''
Solution 1:
A better way of phrasing it might be to use an extra modal expression, as opposed to just the tensed verb were:
As many as 30 people may have been killed in the airstrike.
This would make it clear that you have reason to believe that it's possibe that up to 30 people have been killed, but also that you don't commit yourself to the truth of this alleged fact.