By Michelle Nichols UNITED NATIONS (Reuters) - Israel will be required under international law to take responsibility for helping Palestinian civilians if there are any further large-scale displacements from the fighting in Gaza, a top U.N. envoy told the United Nations Security Council on Thursday. The United Nations is struggling to cope with a flood of some 220,000 Palestinian civilians into shelters. They have come under fire during three weeks of fighting between Israel and Islamist Hamas militants who dominate Gaza. Pierre Krähenbühl, the Swiss-born chief of the U.N. Relief and Works Agency (UNRWA), said the impoverished enclave of 1.8 million Palestinians was facing a precipice, and added that he was alarmed to hear that Israel had warned more neighborhoods in Gaza to evacuate ahead of military action.