By Bassam Masoud and Nidal al-Mughrabi GAZA/CAIRO (Reuters) -Israel pounded the Gaza Strip from north to south on Saturday in an expanded phase of its two-month-old war against Hamas, after the United States wielded its U.N. Security Council veto to shield its ally from a global demand for a ceasefire. Thirteen of the Security Council’s 15 members voted for the resolution calling for an immediate humanitarian ceasefire that was blocked by Washington. Britain abstained. Since a truce collapsed last week, Israel has expanded its ground campaign into the southern half of the Gaza Strip by launchi…