JERUSALEM - Israeli troops and tanks invaded the Hamas-ruled Gaza Strip late yesterday after eight days of punishing airstrikes failed to halt the militant Palestinian group's rocket fire into Israel.
Gunbattles could be heard from Gaza City as 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.
Israeli officials said that they expected a lengthy battle but that they did not intend to occupy 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.
Army ambulances were seen bringing Israeli wounded to a hospital in the southern Israeli city of Beersheba. The military said a total of 30 soldiers were injured in the opening hours of the offensive along with "dozens" of militants.
Gaza residents said troops were seen before dawn today in the town of Beit Lahiya, north of Gaza City, and the sound of intense fighting could be heard just east of the city, toward the border with Israel.
In the city itself, the Hamas-run Al Aqsa radio station was in flames from a missile strike. Staff had evacuated the building about a week earlier, at the start of the Israeli offensive, and continued broadcasting from another location.
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, surrounded in a messy fight with a paramilitary foe, and Palestinian civilian casualties could rise sharply, raising 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 yesterday in Beit Lahiya during late afternoon prayers, killing 13 people and wounding 33 others inside, a Palestinian medical official reported. Two of the dead were children, he said.
The airstrikes 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 said. About one-fourth of them were civilians, according to the United Nations.
Yet the rocket fire by militants in Gaza has continued. Three Israeli civilians and one soldier have been killed in the past week as Hamas deployed more advanced, longer-range projectiles capable of hitting Israeli cities more than 20 miles away.
Hamas fired 29 rockets yesterday before the invasion, 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 against 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. Israeli officials say the group has built a paramilitary force of about 15,000 men, trained them in urban warfare and armed them with a large number of anti-tank missiles and other weapons.
Israeli officials said the aim in Gaza was not so much to eliminate Hamas' capacity to fire rockets but to crush its motivation for doing so. Some Israeli analysts and experts said this could be accomplished by a brief but powerful ground operation.
Israel has resisted international pressure to end the offensive. On Wednesday, Israeli Prime Minister Ehud Olmert rebuffed a French cease-fire proposal.
Since then, diplomatic efforts have focused on the idea of an internationally monitored truce. Late yesterday, the United States blocked approval of a U.N. Security Council statement calling for an immediate cease-fire in the Gaza Strip and southern Israel and expressing concern at the escalation of violence between Israel and Hamas.
Wary of the risks of civilian casualties in Gaza, the Israeli air force dropped thousands of leaflets over Gaza warning people to leave buildings in areas apparently targeted for assault. The military called telephone numbers in Gaza with the same message, pre-recorded in Arabic.
The Associated Press contributed to this article.