The operation also brought Israel its first military casualty. The circumstances behind the death of Staff Sgt. Eitan Barak, 20, were not made clear: Hamas's military wing said it ambushed Israeli units in the northern town of Beit Lahiya, but Israeli media said Barak was likely killed by friendly fire. The army said a number of soldiers were also wounded. Earlier in the week, an Israeli civilian died from Palestinian mortar fire and several others have been wounded.
"The ground offensive does not scare us and we pledge to drown the occupation army in Gaza mud," Hamas spokesman Sami Abu Zuhri said in a statement.
Israeli public opinion strongly supports the offensive after days of unrelenting rocket fire from Gaza and years of southern Israeli residents living under the threat. Gaza militants have fired more than 1,500 rockets at Israel over the past 11 days, and rocket fire continued across Israel Friday.
The order to launch the ground operation was triggered not by the rocket fire, but by a Hamas attempt to infiltrate Israel on Thursday, when 13 armed militants sneaked through a tunnel from Gaza and were killed by an airstrike as they emerged inside Israel.
The military, which has already mobilized more than 50,000 reservists, said paratroopers had uncovered eight tunnel access points across the Gaza Strip and engaged in several gun battles with Hamas militants who ambushed them.
Israeli forces are expected to spend a day or two staking ground within two miles (three kilometers) of the border in the north, east and south of the Gaza Strip. Then, they are expected to begin destroying tunnels, an operation that could take up to two weeks. Tanks, infantry and engineering forces were operating inside Gaza, where the military said it targeted rocket launchers, tunnels and more than 100 other targets.
Hamas has survived Israeli offensives in the past, including a major three-week ground operation in January 2009 and another weeklong air offensive in 2012, but in each case the militant group recovered. It now controls an arsenal of thousands of rockets, some long range and powerful, and it has built a system of underground bunkers.
But Hamas is weaker than it was during the previous two offensives, with little international or even regional support from its main allies, Turkey and Qatar. Protests against the offensive took place Friday in Turkey, Jordan and the West Bank.
Egypt, which has been pushing for a cease-fire, is at odds with Hamas' conditions, which include a lifting of the siege of Gaza and completely open borders into the Sinai — where Egypt is already fighting Islamic extremists.
Heller reported from Jerusalem. Associated Press writers Ian Deitch and Tia Goldenberg and Yousur Alhlou in Jerusalem and Karin Laub in Gaza City contributed to this report.
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