Breaking News: Israeli tanks roll into Gaza

Israeli tanks and infantry entered Gaza after nightfall Saturday, launching a ground offensive that the military said would be a "lengthy operation" in a widening war on Gaza's Hamas rulers. Israeli security officials said the operation is likely to go on for several days, but that the objective is not to reoccupy Gaza. The depth and intensity will also depend on parallel diplomatic efforts, the officials on condition of anonymity in line with military regulations. "We have many, many targets," Israeli military spokeswoman Maj. Avital Leibovich told CNN, adding that Hamas has been digging smuggling tunnels and other facilities. "To my estimation, it will be a lengthy operation," she said. "The goal is to try and take over some of the those launching areas that were responsible for the many launches, thousands of launches in fact, toward Israeli civilians," she said. "The civilians are not our target. We are looking only after militants. Hamas militants." Heavy gun battles were reported as Israeli tanks and infantry soldiers entered Gaza after dark. The forces stayed close to the border area, witnesses said. Heavy artillery fire hit east of Gaza City in areas where Hamas fighters were deployed. A text message sent by Hamas' military wing, Izzedine al-Qassam, said "the Zionists started approaching the trap which our fighters prepared for them." Before the ground operation began, defense officials said around 10,000 soldiers massed along the border in recent days. Heavy artillery fire in the early evening was intended to detonate Hamas explosive devices and mines planted along the border area before troops marched in. It was not immediately clear how deep into Gaza the Israeli forces would go. Israel's offensive against Hamas began with a week of aerial bombardment of Hamas target. However, Hamas kept firing at Israeli towns, and Israeli officials said diplomatic efforts did not produced a satisfactory plan so far to guarantee a halt to rockets. Israel had held off on a ground offensive, in part because of concern about casualties among Israeli troops. Hamas leaders have warned that they have prepared a violent welcome. They have also threatened to resume suicide attacks inside Israel. The Israelis were also backed by helicopter gunships. The first week of fighting had claimed more than 460 Palestinians lives, while four Israelis were killed by rocket fire. Gaza is densely populated, and intense urban warfare was likely to get much deadlier.