An Israeli air strike on a car in Gaza City has killed Hamas military commander Ahmad Jabari and his bodyguard, the Hamas interior ministry says.
Five other Palestinians died in a wave of air strikes on the Gaza Strip on Wednesday, it said.
"The Israeli occupation has carried out more than 20 air strikes on targets and headquarters of the police and security in the Gaza Strip. So far, six people have been killed, including commander Ahmad Jabari," interior ministry spokesman Islam Shahwan said.
Below are reactions from across the world on the assassination and the subsequent air strikes.
Egyptian President Mohamed Morsi has condemned Israel's attacks on the Gaza Strip by calling them "unacceptable" and emphasised they would destabilise the region, during in a televised address to the nation.
Morsi’s lambasted the Jewish state on Thursday for the first time since he took office in June following the overthrow of Hosni Mubabrak.
"We are in contact with the people of Gaza and with Palestinians and we stand by them until we stop the aggression and we do not accept under any circumstances the continuation of this aggression on the Strip," Mursi said.
"The Israelis must realise that this aggression is unacceptable and would only lead to instability in the region and would negatively and greatly impact the security of the region," he said.
Egypt's Foreign Minister Mohammed Kamel Amr condemned the Israeli air strikes that killed Jabari and called for an "immediate stop" to attacks on the Palestinian territory.
Amr, in a statement, warned Israel against any "escalation and its possible negative effects on regional stability".
The movement's Freedom and Justice Party (FJP) warned that Egypt would no longer stand by as Palestinians come under attack.
The FJP said the air strikes that killed Jabari in Gaza required "swift Arab and international action to stop the massacres".
UN chief Ban Ki-moon called on Israel and Hamas fighters to prevent an escalation of hostilities in the Gaza Strip, and urged both sides to ensure civilians will be protected, a UN spokesman said.
"The Secretary-General calls for an immediate de-escalation of tensions," UN spokesman Martin Nesirky said, adding that it was a reiteration of Ban's already-stated position.
"Both sides should do everything to avoid further escalation and they must respect their obligations under international humanitarian law to ensure the protection of civilians at all times," he told reporters.
Qatari Foreign Minister Hamad bin Jassim Al Thani condemned the attacks in the Gaza Strip and said they should not go unpunished.
"I condemn in the name of Qatar... This filthy crime must not pass without a punishment," he told journalists after a meeting in the Saudi capital between Gulf Co-operation Council Foreign Ministers and their Russian counterpart Sergei Lavrov to discuss Syria.
"The UN Security Council must shoulder its responsibilities in preserving peace and security in the world."
Middle East peace envoy Tony Blair called on Hamas on Thursday to stop firing rockets from Gaza into Israel, saying that as long as it continued the Jewish state would act to defend itself.
"If the rocket fire carries on out of Gaza, targeted at Israeli towns and villages, then the retaliation will increase," the former British prime minister told a UK televion news network.
"There are roughly a million people who take shelter every night (in Israel). There's no government where its citizens are under that pressure that isn't itself going to come under pressure to take action," Blair said.
"Likewise I understand the pressures on the Hamas leadership but the answer to those pressures I'm afraid is not to keep firing rockets. It's to find a different and better way forward."
Blair, who represents the diplomatic Quartet comprising the United Nations, United States, European Union and Russia, said the immediate priority was to calm the situation to stop it destabilising the region.
The United States threw its weight behind Israel after it launched a barrage of air strikes on the Gaza Strip, and condemned Palestinian militant rocket attacks on southern Israel.
"We support Israel's right to defend itself, and we encourage Israel to continue to take every effort to avoid civilian casualties," US State Department spokesman Mark Toner said in a statement.
"We strongly condemn the barrage of rocket fire from Gaza into southern Israel, and we regret the death and injury of innocent Israeli and Palestinian civilians caused by the ensuing violence," he added.
"There is no justification for the violence that Hamas and other terrorist organizations are employing against the people of Israel," Toner said in his statement, calling on them "to stop these cowardly acts immediately."
"Hamas claims to have the best interests of the Palestinian people at heart, yet it continues to engage in violence that is counterproductive to the Palestinian cause," he added.
"Attacking Israel on a near daily basis does nothing to help Palestinians in Gaza or to move the Palestinian people any closer to achieving self determination."