GAZA CITY, Gaza Strip — 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.

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.

"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.

She said the goal of the operation was to take over the areas used by militants to launch rockets against Israel.

"The civilians are not our target, only militants, Hamas militants," she said.

Defense officials have 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.

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.