GAZA CITY (Palestinian Territories) - An Israeli air strike killed a senior Hamas military commander in Gaza City on Wednesday, prompting a furious response from militants who said it had opened "the gates of hell."

Israel said the killing of Ahmed Jaabari was the beginning of an operation to target militant groups across the Hamas-controlled territory.

The assassination follows a flareup in and around the Gaza border, which saw militants fire more than 120 rockets over the border, and Israel killing seven Palestinians.

The killing sparked furious protests in Gaza City, with hundreds of members of Hamas and the Ezzedine al-Qassam Brigades chanting for revenge inside Shifa hospital.

Witnesses reported at least six air strikes across the territory and said dozens of Israeli tanks were massed along the border east of Gaza City.

Hamas's armed wing, the Ezzedine al-Qassam Brigades, warned that "the occupation has opened the gates of hell on itself."

And Hamas issued a statement saying Israel would be responsible for the consequences of the assassination.

"The Palestinian government mourns the loss of the senior Palestinian leader Ahmed al-Jaabari, one of the symbols of the resistance and holds the occupation responsible for the consequences of this crime," a statement said.

Medical staff at the Shifa hospital in Gaza City also confirmed his death, saying his bodyguard was also killed in the strike.

Military spokeswoman Avital Leibovich said the strike was just the start of an operation targeting armed groups in Gaza following multiple rocket attacks on southern Israel.

"After the rocket fire of recent days, the chief of staff (Lieutenant General Benny Gantz) has decided to authorise the targeting of terrorist organisations in the Gaza Strip -- Hamas, Islamic Jihad and others," she told reporters, adding: "This is the beginning."

Israel's Shin Bet domestic intelligence agency and the military confirmed it was a targeted killing.

"During a joint operation of the General Security Service (Shin Bet) and the IDF (army) today, Ahmed Jaabari, the senior commander of the military wing of Hamas in the Gaza Strip, was targeted," a Shin Bet statement said.

"In the past hour, the IDF targeted Ahmed Jaabari, the head of Hamas's military wing, in the Gaza Strip," an army statement said, describing Jaabari as "a senior Hamas operative... directly responsible for executing terror attacks."

"The purpose of this operation was to severely impair the command and control chain of the Hamas leadership, as well as its terrorist infrastructure," it said.

Outside the hospital, armed men fired weapons into the air, and mosques throughout the city called prayers to mourn the commander's death.