Al-Quds Brigades, the armed wing of Palestinian group the Islamic Jihad, said on Saturday that its fighters had clashed with Israeli special forces trying to advance into Gaza.
The Brigades said there were "losses" among attacking Israeli forces and that it would unveil details later.
Israel's military commander Gantz said his forces were ready to act as needed - an indication of a readiness to send in tanks and other ground troops, as Israel last did for two weeks in early 2009.
“We are in the midst of an assault and we are prepared to expand it as much as is required, to wherever is required, with whatever force will be required and for as long as will be required,” Gantz told reporters.
Some 20,000 reservists have already been mobilized, the army said.
Prime Minister Benjamin Netanyahu said Israel has attacked more than 1,000 targets during the four-day-old offensive militants and that “there are still more to go.”
In remarks to reporters, he said he saw no international pressure on Israel to halt its campaign.
He also would not rule out the possibility of expanding the campaign of mostly aerial attacks into a ground war in Gaza, answering when asked whether such a move was possible that “we are weighing all possibilities and preparing for all possibilities.”
Hamas's armed wing Azzeddine el-Kasam threatened on Friday to fire rockets at Tel Aviv's Ben-Gurion international airportand warned airlines not to fly to Israel's main gateway to the world.
The rocket salvoes by the hardline movement and its allies, some striking more than 100 km (60 miles) from Gaza, have killed no one so far, due in part to interception by Israel's partly-U.S. funded Iron Dome aerial defense system.
Meanwhile, the death toll in the blockaded enclave of Gaza has exceeded 100, most of them civilians, according to Palestinians officials.
Israel said the offensive is meant to end cross-border rocket attacks that intensified last month after its forces arrested hundreds of activists from Hamas following the abduction there of three Jewish teenagers who were later found killed.
A Palestinian youth was reportedly burned alive in Jerusalem in a suspected Israeli revenge attack.
“Terrorists in Gaza made a grave mistake by attacking the people of Israel. They are bringing disaster upon themselves,” army Chief-of-Staff Lt.-Gen. Benny Gantz said in a statement posted on Twitter on Friday.
Amid growing international outcry over the civilian rising civilian casualties among Palestinians in Gaza, they Israel army issued a statement saying: “When houses are used for military purposes, they may become legitimate military targets under international law.”
Navi Pillay, the U.N. high commissioner for human rights, had called on the Israeli military to abide by international law.
“We have received deeply disturbing reports that many of the civilian casualties, including of children, occurred as a result of strikes on homes,” Pillay said. “Such reports raise serious doubt about whether the Israeli strikes have been in accordance with international humanitarian law and international human rights law.”
Pillay said she's seen for herself in Gaza how traumatic the airstrikes and rocket attacks are for civilians, particularly children. Her office said civilians bear the brunt of the conflict now - and all sides must refrain from launching attacks or putting military weapons in densely populated areas.
“Israel, Hamas, and Palestinian armed groups in Gaza have been down this road before, and it has led only to death, destruction, distrust and a painful prolongation of the conflict,” Pillay said.
[With Reuters and AFP]