Israel killed a top Hamas military commander in a targeted air strike in Gaza on Wednesday, prompting outrage from Palestinian militants who said the Jewish state had opened "the gates of hell."

And Egypt promptly announced it was recalling its ambassador over Israel's hit on a car in Gaza City that killed Ahmed Jaabari and his bodyguard, swiftly followed by more than 20 air strikes in Gaza that killed another five people and wounded 60.

But Israel warned it was only the start of an operation targeting militant groups in Gaza, which comes as the Jewish state prepares for general elections in January.

"Today we sent a clear message to Hamas and other terrorist organisations," Israeli Prime Minister Benjamin Netanyahu said during a televised address several hours after the initial strike which took place at around 1400 GMT.

"If it becomes necessary, we are prepared to expand the operation," the Israeli leader warned after holding consultations with his security cabinet. "We will not tolerate a situation in which Israeli citizens are threatened by rocket fire."

Defence Minister Ehud Barak said the operation was aimed at strengthening Israel's deterrence, damaging militant groups' rocket-firing capabilities and stamping out attacks on southern Israel.

"Israel doesn't want a war but the Hamas provocation of recent weeks... forced us to act sharply and decisively," Barak said.

"We are at the beginning, not end of this action," he warned, adding: "It won't be a quick fix."

Following the strike that killed Jaabari and his bodyguard, Israel launched at least 20 raids, killing another five people, including two young children, and wounding at least 60, Hamas health ministry spokesman Ashraf al-Qudra said.

"We recommend that no Hamas terrorists, low level or senior leaders, show their faces above ground in the days ahead," wrote Netanyahu spokesman Ofir Gendelman on his official Twitter account.

The air strikes capped five days of rising tension in and around Gaza, which saw Israel kill seven Palestinians and militants fire more than 120 rockets over the border, injuring eight.

Jaabari's death sparked furious protests in Gaza City, with hundreds of members of Hamas and its armed wing, the Ezzedine al-Qassam Brigades, chanting for revenge in and around Shifa hospital where Jaabari's body was taken.

The Israeli army said it had also targeted "a significant number of long-range rocket sites" and was prepared to launch a ground operation if necessary in order to stamp out rocket fire.

"All options are on the table. If necessary, the IDF (army) is ready to initiate a ground operation in Gaza," the military said on its official Twitter account.

Police spokeswoman Luba Samri said that 55 rockets had been fired at Israel from Gaza in the aftermath of the Gaza City strike and that the level of alert had been raised across the area.

In a separate statement, the military said its vaunted Iron Dome anti-missile system had successfully intercepted 13 of the rockets.

The Israeli strikes prompted widespread condemnation, with Egyptian President Mohamed Morsi recalling Cairo's envoy to Israel and summoning Israel's ambassador, his spokesman said.

--'Gates of hell' --

Gaza's Hamas government welcomed the Egyptian move in a statement which also urged Arab leaders "to take decisive action" to push Israel to end its campaign.

And following a request from Morsi, Arab League chief Nabil al-Arabi said top Arab diplomats were preparing to hold an emergency meeting on Saturday to discuss the violence.

Britain also urged restraint and Russia said it was "very concerned," while Washington said it was watching developments in Gaza "closely."

The strike in Gaza City prompted an outpouring of grief and anger, with armed men firing weapons into the air outside Shifa hospital and mosques throughout the city calling prayers to mourn the commander's death.

The Qassam Brigades, in a furious statement, said Israel had "opened the gates of hell on itself," while Hamas spokesman Fawzi Barhum said the strike was tantamount to a "declaration of war."

"The occupation committed a dangerous crime and crossed all the red lines, which is considered a declaration of war," he said in a statement. "The occupation will pay dearly for this."

Israel's last major operation in Gaza began at the end of December 2008, six weeks shy of general elections, when troops embarked on a 22-day campaign which killed 1,400 Palestinians -- half of them civilians -- and 13 Israelis, 10 of them soldiers.