Israeli troops have pushed deeper into Gaza to destroy rocket launching sites and tunnels, firing volleys of tank shells and clashing with Palestinian fighters in a high-stakes ground offensive meant to weaken the enclave's Hamas rulers.
The assault has opened a new, potentially extended and bloodier stage in the conflict after a 10-day Israeli campaign of more than 2,000 air strikes against Gaza that had failed to halt relentless Hamas rocket fire on Israeli cities.
Prime minister Benjamin Netanyahu said he ordered his military to prepare for a possible "significant" expansion of the operation.
The government said its goal is to stop rocket attacks, destroy the network of Hamas tunnels into Israel and weaken Hamas militarily, but there are calls from hardliners in Israel to completely crush Hamas and drive it from power in Gaza.
That could mean a longer operation with the danger of mounting casualties in a conflict that has already seen more than 274 Palestinians killed in Gaza, around a fifth of them children.
Israel had been reticent about launching a ground offensive for fear of endangering its own soldiers and drawing international condemnation over mounting Palestinian civilian deaths.
But after an attempt by Hamas to infiltrate Israel yesterday, when 13 armed militants sneaked through a tunnel from Gaza, only to be killed by an air strike as they emerged inside Israel, Mr Netanyahu gave the order that evening for thousands of troops on standby to enter Gaza.
"We chose to begin this operation after the other options were exhausted and with the understanding that without the operation the price we will pay can be very high," he said before a special cabinet meeting in Tel Aviv.
He said his instructions for the military were to "be ready for the possibility of a significant expansion of the ground operation".
Israel saw its first military death of the conflict in the early hours of the ground assault. The circumstances behind the death of Staff Sergeant Eitan Barak, 20, were not clear. Hamas's military wing said it ambushed Israeli units in the northern town of Beit Lahiya, but Israeli media said Sgt Barak was likely to have been killed by friendly fire. An Israeli civilian died from mortar fire earlier in the week, and several have been wounded.
The Israeli military said it had killed nearly 20 militants in exchanges of fire. Gaza health officials said 25 Palestinians had been killed since the ground operation began, including three teenage siblings from the Abu Musallam family who were killed when a tank shell hit their home. At the morgue, one of the victims' faces was blackened by soot and he and his siblings were each wrapped in a white burial shroud.
Their father Ismail said the three were sleeping when the shell struck, and he had to dig them out from under the rubble.
Israel says it is going to great lengths to avoid civilian casualties and blames them on Hamas, accusing it of firing from within residential neighborhoods and using its civilians as "human shields".
The streets of Gaza City were largely deserted, though some roadside vegetable vendors remained open. The sound of steady shelling could be heard across Gaza as Israel continued to strike targets from the air, and buildings shook as missiles hit.
"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.
Hamas has survived Israeli offensives in the past, including a major three-week ground operation in January 2009 from which it emerged militarily weaker, but in each case it recovered. The group 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 now than it was during the previous two offensives - in 2008-09 and 2012 - with little international or even regional support, with its main allies Turkey and Qatar largely sidelined by others in the region. Protests against the offensive took place today in Jordan, Turkey and the West Bank.
Israeli defence officials said soldiers faced little resistance during the first night of the ground operation. The military said paratroopers had already uncovered eight tunnel access points across the Gaza Strip and engaged in several gun battles with forces that ambushed them.
Forces are expected to spend a day or two staking ground within two miles of the border and are working in the north, east and south of the Gaza Strip. Then, they are expected to move to the second phase, which is to destroy tunnels, an operation that could take up to two weeks.
Tanks, infantry and engineering forces were operating inside the coastal strip. The military said it targeted rocket launchers, tunnels and more than 100 other targets, and that a number of soldiers were wounded.
Israeli public opinion appears to strongly support 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. Rocket fire continued across Israel today, with one exploding near a nursery south of Tel Aviv, lightly wounding a woman nearby, the military said.