Israeli Prime Minister Benjamin Netanyahu said the army was hitting Hamas hard with what he called surgical strikes, and warned of a "significant widening" of the Gaza operation. Israel will "continue to take whatever action is necessary to defend our people," said Netanyahu, who is up for re-election in January.
At least 12 trucks were seen transporting tanks and armored personnel carriers toward Gaza late Thursday, and buses carrying soldiers headed toward the border area. Israeli TV stations said a Gaza operation was expected on Friday, though military officials said no decision had been made.
"We will continue the attacks and we will increase the attacks, and I believe we will obtain our objectives," said Lt. Gen. Benny Gantz, Israel's military chief.
An Israeli ground offensive could be costly to both sides. In the last Gaza war, Israel devastated large areas of the territory, setting back Hamas' fighting capabilities but also paying the price of increasing diplomatic isolation because of a civilian death toll numbering in the hundreds.
The current round of fighting is reminiscent of the first days of that three-week offensive against Hamas. Israel also caught Hamas off-guard then with a barrage of missile strikes and threatened to follow up with a ground offensive.
However, much has also changed since then.
Israel has improved its missile defense systems, but is facing a more heavily armed Hamas. Israel estimates militants possess 12,000 rockets, including more sophisticated weapons from Iran and from Libyan stockpiles plundered after the fall of Moammar Gadhafi's regime there last year.
Netanyahu, who has clashed even with his allies over the deadlock in Mideast peace efforts, appears to have less diplomatic leeway than his predecessor, Ehud Olmert, making a lengthy military offensive harder to sustain.
What's more, regional alignments have changed dramatically since the last Gaza war. Hamas has emerged from its political isolation as its parent movement, the region-wide Muslim Brotherhood, rose to power in several countries in the wake of last year's Arab uprisings, particularly in Egypt.
Egypt recalled its ambassador to protest the Israeli offensive and has ordered his prime minister to lead a senior delegation to Gaza on Friday in a show of support for Hamas.
At the same time, while relations with Israel have cooled since the toppling of longtime Egyptian leader Hosni Mubarak, Morsi has not brought a radical change in Egypt's policy toward Israel. He has promised to abide by Egypt's 1979 peace deal with Israel and his government has continued contacts with Israel through its non-Brotherhood members.