The US has said the shelling of a UN shelter in Gaza is "totally unacceptable and totally indefensible". In its strongest criticism yet of Israel's offensive in the Palestinian territory, the US - Israel's closest ally - also said the civilian casualties were "too high". It urged Israel to do more to protect civilian life. Israel says its offensive in Gaza is designed to defend its population from attacks by Palestinian militants. The UN has also criticised Israel over the worsening situation in Gaza, saying people there are "facing a precipice".