JERUSALEM — Israeli troops and tanks invaded the Hamas-ruled Gaza Strip late Saturday after eight days of punishing airstrikes failed to halt the militant Palestinian group's rocket fire into Israel.
Gun battles and explosions could be heard from Gaza City as high-rise buildings shook and artillery rounds lighted the night sky. Columns of tanks and infantry, backed by helicopter gunships, pushed nearly half a mile into the territory from three directions.
Medical authorities in Gaza reported today that five militants and three civilians were killed in the early hours of ground fighting. Israel said 30 of its soldiers and "dozens" of militants were wounded.
Israeli officials said they expected a lengthy battle but did not intend to remain in Gaza.
"This will not be easy and it will not be short," Israeli Defense Minister Ehud Barak said.
Hamas issued a defiant statement saying Gaza would "become a graveyard" for Israeli soldiers. Later, its radio and TV stations went off the air, apparently disrupted by the Israelis.
The ground offensive, involving thousands of soldiers, was aimed primarily at Hamas' rocket-launching facilities, Israeli officials said. Some of those sites are in open fields, but many are hidden across Gaza in densely populated areas and are difficult to pinpoint from the air.
In choosing to strike from the ground as well as the air, Israel undertook two risks: Its army could get bogged down in a messy fight with a determined paramilitary foe. And Palestinian civilian casualties could rise sharply, increasing international pressure on Israel to halt the operation.
Israel's airstrikes have already taken a heavy civilian toll. A missile demolished part of a mosque Saturday in the northern town of Beit Lahiya during late afternoon prayers, killing 13 people and wounding 33 inside, a Palestinian medical official reported. Two of the dead were children, he said.
The strikes began Dec. 27, a week after Hamas let an Egypt-brokered truce lapse. The six-month cease-fire had begun to break down in November.
More than 460 Palestinians have been killed in the operation, Palestinian officials say. About one-fourth of them were civilians, according to the United Nations' tallies.
Yet the rocket fire by Gaza militants has continued. Three Israeli civilians and one soldier have been killed in the last week, as Hamas deployed more advanced, longer-range projectiles capable of hitting Israeli cities more than 20 miles away.
Hamas fired 29 rockets Saturday before the ground invasion began, Israeli officials said, hitting four homes in southern Israel and wounding three people.
In their defiance, Hamas' Islamist leaders appear to be gambling that they can withstand an onslaught by the Middle East's mightiest army and remain in power in Gaza.
Hamas, an Islamic group backed by Iran and whose charter calls for the Jewish state's destruction, won the Palestinian Authority's parliamentary elections in early 2006. An attempted unity government with the more secular, moderate Fatah collapsed in June 2007 amid fighting in which Hamas seized full control of Gaza. Afterward, Israel tightened a blockade of the territory.
Israel had unilaterally withdrawn its troops and settlers from the coastal strip in 2005, while retaining control of air, sea and most land access. But its own border communities continued to come under frequent attack by crude rockets fired by Hamas and smaller Palestinian militant groups.
Hamas has been preparing for an all-out Israeli invasion for more than a year. Israeli officials say the group has built a paramilitary force of about 15,000 men, trained in urban warfare and armed with a large number of anti-tank missiles and other weapons.
During the recent cease-fire, Hamas used smuggling tunnels under the border with Egypt to expand its arsenal and fortify its military posts, including a network of underground bunkers, Israeli officials said. Some, but not all, of that infrastructure was damaged by the bombings, they said.
Israel's ground offensive began after dark following three hours of artillery fire aimed at blowing up mines Hamas is believed to have placed along the border.
The initial clashes took place in open fields, Israeli officials said. The soldiers did not immediately move into Gaza's crowded cities, but some were seen before dawn today in Beit Lahiya.
The soldiers wore night-vision goggles on their helmets and camouflage paint on their faces.
Israel's Channel 10 television said several thousand gunmen representing all of Gaza's smaller militant factions had joined in the fighting.
Military analysts said they expected Hamas to seek to draw the fight to the 140-square-mile territory's urban areas.
"Our soldiers will face small cells that know they cannot match the army's force," Shmuel Zakai, a retired brigadier general and former commander of the army's Gaza division, told Channel 10 television. "Hamas will use other tactics to try to inflict as many casualties as possible."