An Israeli mobile artillery unit fires towards the Gaza Strip July 18, 2014. Photo: Reuters
Israel is prepared to "significantly widen" its ground offensive against militants in the Gaza Strip, said its Prime Minister Benjamin Netanyahu.
He said the military was targeting the Hamas tunnel network, which it could not do "only from the air".
Thousands of troops moved into parts of Gaza on Thursday night, backed by tanks and artillery fire.
Hamas, the Palestinian group that controls Gaza, has said Israel will "pay a high price" for the invasion.
The decision to send in troops came after 10 days of intensive rocket fire and air strikes between the two sides.
"My instructions... are to prepare for the possibility of significantly widening the ground operation, and the military is preparing accordingly," he told reporters before a cabinet meeting on Friday.
At least 24 Palestinians and one Israeli soldier have been killed since the ground offensive began on Thursday.