Israel has launched a large-scale ground offensive in the Gaza Strip, escalating a 10-day military operation to try to destroy Hamas' weapons arsenal, rocket firing abilities and tunnels under the Palestinian territory's border with Israel.
It was the first major Israeli ground offensive in Gaza in just over five years. It came hours after Hamas militants tried to infiltrate Israel through a tunnel under the Gaza-Israel border, but were stopped by Israel.
Lt Col Peter Lerner, an Israeli military spokesman, said the operation is open-ended.
"We will be striking the infrastructure," he said. "We will be striking the operatives in order to safeguard the civilians of the state of Israel especially issues to do with tunnelling, that was exemplified earlier today."
Thousands of Israeli soldiers had massed on the border with Gaza in recent days, waiting for the order to go in.
The operation also followed a brief truce earlier in which Israel held fire to allow Gazans to stock up on food and other necessities after being largely holed up at home since the conflict began last month.
Israeli strikes have hit more than 2,000 targets in Gaza and Hamas launched nearly 1,500 rockets at Israel, the Israeli military has said.
Israeli Prime Minister Benjamin Netanyahu instructed the army to go ahead after Hamas rejected an Egyptian ceasefire plan earlier in the week and after Hamas militants tried to infiltrate into Israel through a tunnel from Gaza.
"In light of the despicable and relentless aggression by Hamas and the dangerous infiltration into Israel, Israel is obliged to protect its citizens," the statement said.
Israel last carried out a major ground offensive in Gaza in January 2009.
Earlier an Israeli air strike killed three children from the same family in Gaza City.
Two boys and a girl between the ages of eight and 10 were killed when their home was hit.
Their grandfather, Marzouk Shahaibar, said they had gone on the roof of the home to feed pigeons. TV footage from the scene showed a doll and a sandal near pools of blood on the roof.
The deaths come a day after four cousins aged nine to 11 were killed on the beach beside a coastal road west of Gaza City.