PALMACHIM AIRBASE, Israel (Reuters) -Heavy civilian casualties are the cost of Israel's intense campaign to destroy Hamas in Gaza and the militants' urban warfare strategy, Israeli military officials said, in the face of global alarm at the staggering toll from the bombing. Israel has dropped thousands of tonnes of munitions over the past 10 weeks, leaving the narrow Mediterranean strip in ruins and killing nearly 20,000, with more believed to be trapped under collapsed buildings, Gazan official