当然是 it was ；
than 在这里是连词,后面接 it was reported that there were ... ；
其实这一句的意思也可以这样表达：It was reported that there were some casulties, but actually there were more.；
what 在作为关系代词引导定语从句时,相当于“先行词 + 关系代词”；
在这一句当中,因为前面的 casuslties 是复数,what 则相当于 those which / that ,根据主谓语一致的原则,原句应为：there were more casualties than were reported；
句子的意思是：伤亡人员比报道的要多.