By Bassam Masoud, Nidal al-Mughrabi and Michelle Nichols GAZA/CAIRO/NEW YORK (Reuters) – The United States kept up pressure on Israel to do more to protect Palestinian civilians during a fierce offensive against Hamas militants across Gaza, even as Washington vetoed a U.N. Security Council demand for an immediate ceasefire. Fighting escalated and the Palestinian death toll rose on Friday, with Israel pounding the enclave from north to south in an expanded phase of the two-month-old war against the Islamist group Hamas. Decrying a “spiralling humanitarian nightmare”, U.N. Secretary-General Anto…