By Mohammed Salem, Simon Lewis and Nidal al-Mughrabi GAZA/RAMALLAH/DOHA (Reuters) -Israeli strikes in southern and central Gaza intensified on Wednesday despite a pledge by Israel that it would pull out some troops and shift to a more targeted campaign, and pleading from its ally Washington to kill fewer civilians. In the latest sign of the three-month-old war spreading, U.S. and British warships in the Red Sea fended off the biggest attack yet from Yemen’s Houthi movement, which says it is acting to support Gaza. Washington and London said they shot down 21 drones and missiles aimed at shippi…