Jabari played a key role in Hamas' takeover of the Gaza Strip in 2007 and in last year's release of captured Israeli soldier Gilad Shalit. Israeli officials say he also masterminded attacks on Israelis.
"He has a lot of blood on his hands," Leibovitz said.
The strike was followed by nearly two dozen Israeli airstrikes throughout the territory, focusing on rocket launchers, long-range rocket caches and other weaponry.
Leibovitz would not say how long the operation was expected to last or whether it would include ground operations. Military officials said ground forces were standing by. "Airstrikes might not be sufficient," one military official said.
Analysts said Israeli leaders would prefer to avoid a major ground assault, but the decision will depend on how Hamas reacts.
"I think we are witnessing a larger operation that will probably be ongoing in the next few days," said Maj. Gen. Israel Ziv, a former commander of the IDF's Gaza division and now a military analyst.
"It's probably a big shock for Hamas to get such a hit," he said. "I assume there's going to be an escalation in the next few days."
Hamas leaders immediately vowed to retaliate, saying the killing of Jabari would take hostilities with Israel to a new level.
"The assassination of the commander Ahmed Jabari is tantamount to a declaration of war, and the occupation will pay a heavy price for it," Hamas said in a statement.
At least seven other people were killed and 25 were wounded, Gaza hospital officials said.
Minutes after Jabari's killing, Palestinian militants resumed firing rockets into southern Israeli towns. But many of the rockets were blocked by Israel's missile defense shield known as Iron Dome.
School in southern Israel has been canceled indefinitely and the government approved the calling up of army reserves.
After conferring with his Cabinet on Wednesday night, Netanyahu said Israel would no longer tolerate the rocket attacks.
"Today we delivered a clear message to Hamas and other terror organizations, and if need be, the IDF is prepared to expand the operation, and we will continue to do everything to defend our citizens."
Special correspondents Rushdi Abu Alouf in Gaza City and Reem Abdellatif in Cairo and Times staff writer Jeffrey Fleishman in Sana, Yemen, contributed to this report.