JERUSALEM,— Israeli forces launched a ground assault on Gaza City on Tuesday, advancing street by street through the battered territory as thousands of residents fled with their possessions. An Israeli military officer, speaking anonymously under army regulations, said troops had begun the “main phase” of the offensive, moving from the outskirts toward the center. The officer estimated between 2,000 and 3,000 Hamas fighters remain in Gaza City, mostly operating in small units. By its conclusion, an Israeli military presentation indicated, troops intend to hold much of Gaza, leaving only a stretch of coastal land outside their control. Before the evacuation orders, Gaza City and surrounding areas were home to about one million people, though the Israeli army estimates roughly 350,000 have left.