The Egyptian Muslim Brotherhood's Freedom and Justice Party has condemned the Israeli airstrike that killed Ahmed Jabari, head of the Hamas military wing.
Egypt "will not allow the Palestinians to be subjected to Israeli aggression as in the past," the party, reportedly close to Hamas, said.
Meanwhile Palestinian president Mahmoud Abbas has called for an urgent Arab League meeting over the latest wave of strikes to hit the Gaza strip.
"Barakat al-Fara, the Palestinian ambassador in Cairo and the Palestinian representative in the Arab League, announced that based on instructions from President Mahmoud Abbas, the Palestinian state had asked for an urgent meeting of the Arab League to discuss the Israeli offense on Gaza strip," Egypt's news agency MENA said.
IDF military spokeswoman Avital Leibovitch said the operation - more than 20 strikes - targeted "specific terror sites" used to store rockets for use against Isralis.
"We have started an operation directed toward terror targets in the Gaza Strip following a very long year of rocket launches... The first target was [Jabari]," Leibovitch told BBC.
"There are 1.5 million people in Gaza, and even though we were targeted on a daily basis, we still helped Gazans in our hospitals because we are humanitarian people," she told Al Jaazera. "All options are on the table for us. We will do whatever necessary to protect the people of Israel".
The spokesman for the Hamas Interior ministry, Islam Shahwan, told AFP that six Palestinians have been killed thus far.
Palestinian witnesses also reported an explosion in the house of Hamas co-founder Mahmoud al-Zahar in Gaza.