The fighting, the heaviest in four years, came after Israel launched a ferocious air assault Wednesday to stop repeated rocket fire from Gaza. The Hamas military chief was killed in that strike, and 18 other Palestinians have died over two days, including five children. About 100 Palestinians have been wounded.
Israeli warplanes struck dozens of Hamas-linked targets in Gaza on Thursday, sending loud booms echoing across the narrow Mediterranean coastal strip at regular intervals, followed by gray columns of smoke. After nightfall, several explosions shook Gaza City several minutes apart, a sign the strikes were not letting up, and the military said the targets were about 70 underground rocket-launching sites.
Israeli Prime Minister Benjamin Netanyahu said the army was hitting Hamas hard with what he called surgical strikes, and he 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 reelection in January.
There were mounting signs of a ground operation. At least 12 trucks were seen transporting tanks and armored personnel carriers toward Gaza late Thursday, and a number of buses carrying soldiers arrived. Israeli TV stations said a Gaza incursion was expected Friday, though military officials said no decision had been made.
Defense Minister Ehud Barak said he authorized the call-up of reservists, and the army said up to 30,000 additional troops could be drafted.
"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.
Hamas, meanwhile, warned it would strike deeper inside Israel with Iranian-made Fajr-5 rockets, acknowledging for the first time it has such longer-range weapons capable of hitting targets 47 miles away. Tel Aviv is 40 miles from Gaza.
By nightfall Thursday, Hamas said it had fired more than 350 rockets into Israel. Israel, which estimates Gaza militants have as many as 12,000 rockets, said that about 220 rockets struck the Jewish state and that 130 were intercepted by an antimissile shield.
Israel believes Hamas has significantly boosted its arsenal since the last Gaza war four years ago, including with weapons from Iran and from Libyan stockpiles plundered after the 2011 fall of the regime there.
"After four years, we became stronger, we have a strategy, and we became united with all the military wings in Gaza," said Hamas spokesman Fawzi Barhoum, referring to Hamas' setbacks during Israel's last major offensive in late 2008.
In the current round of fighting, Israel is facing an emboldened Hamas with a stronger arsenal and greater regional backing. Egyptian President Mohammed Morsi, like Hamas a member of the region-wide Muslim Brotherhood, said he was sending a high-level delegation to Gaza on Friday in a show of support for the fellow Islamists there.
Both Israel and Hamas had largely observed an informal truce over the last four years, marred by occasional flare-ups. In recent days, however, border tensions escalated, then exploded into major violence Wednesday when Israel assassinated Hamas' secretive military chief, Ahmed Jabari, with a missile strike on his car.
Jabari led Hamas' 2007 takeover of the territory, turning small squads of Hamas gunmen into a fighting force and supervising Gaza's fledgling arms industry, including rocket production. He was long No. 1 on Israel's most-wanted list, particularly for his role in capturing Israeli Sgt. Gilad Schalit and holding him for more than five years.
On Thursday, Hamas fighters fired machine guns in the air as mourners carried Jabari's body, wrapped in a white burial shroud, through the streets of Gaza City. At the cemetery, young men surged toward the corpse, trying to touch Jabari's face before he was lowered into the grave.
Hamas' top leaders have dropped out of sight since the assassination, but it was not clear if they would be targets. The Hamas prime minister, Ismail Haniyeh, said in a televised speech Thursday that the group "will not forget and not forgive" the killing of Jabari.
Late Thursday, Hamas security said an Israeli navy vessel fired toward a building about 50 yards from Haniyeh's house.
In Israel, a rocket hit a four-story apartment building Thursday in the southern town of Kiryat Malachi, killing two men and a pregnant woman, Mira Scharf, 26, a mother of three. Israeli media said she had recently returned to Israel from New Delhi to give birth.
The White House came out in support of Israel on Thursday, with spokesman Jay Carney saying there is "no justification" for rocket fire from Gaza.