Two were killed in a strike that hit a charitable association for the disabled in Beit Lahiya in northern Gaza, while three others died in a second attack in western Gaza City, local health ministry spokesman Ashraf al-Qudra said.
A defiant Hamas fired five more rockets into Israel as the Islamist movement rejected growing international calls for a halt to hostilities, insisting Israel must act first.
“No international pressure will prevent us from striking, with all force, against the terrorist organisation which calls for our destruction,” he said.
“No terrorist target in Gaza is immune.”
Despite international concern, truce efforts have been unsuccessful, according to Egypt, which has been key in mediating previous ceasefires between Hamas and Israel.
“Egypt has communicated with all sides to halt violence against civilians and called on them to continue with the truce agreement signed in November 2012,” the foreign ministry said.
“Unfortunately, these efforts … have met with stubbornness.”
After weeks of rocket fire into its southern flank, Israel appeared bent on dealing a fatal blow to Hamas.
Ismail Haniya, Gaza’s former premier and the most senior Hamas official in the coastal enclave, ruled out any halt to hostilities.
“(Israel) is the one that started this aggression and it must stop, because we are (simply) defending ourselves,” he said.
Israel says preparations are under way for a possible ground attack, with tanks and artillery massed along the border and some 33,000 reservists mobilised out of 40,000 approved by the cabinet.
More armour was seen heading south on Saturday morning.
Foreign Minister Avigdor Lieberman said he expected a political decision on a possible ground operation by Sunday.
“At the moment we are dealing with the first phase … air attacks,” he told Channel One television.
“I imagine we shall decide tomorrow (Saturday) or the day after on the next stage.”
Early on Saturday, 16 Palestinians died in a wave of Israeli air strikes as Operation Protective Edge entered its fifth day, taking the overall death toll to 121, medics said.
The Israeli military said it had struck “several terrorists conspiring to launch rockets at Israel” and “a weapons cache concealed within a mosque in the central Gaza Strip.”
So far, no one in Israel has been killed. Two have been seriously wounded, including a man at a petrol station hit by a rocket.
In northern Israel, at least one rocket fired from Lebanon struck an open area near the town of Metula on Friday, prompting troops to respond with shelling, the army said.
The military believed a Palestinian group fired it in solidarity with Hamas, public radio reported.
The escalating violence brought more offers of truce negotiations from the White House on Friday.
“There are a number of relationships the United States has that we are willing to leverage in the region to try to bring about an end to the rocket fire that’s originating in Gaza and, as we saw this morning, in Lebanon,” said White House spokesman Josh Earnest on Friday.
He referred to taking steps as the US and Egypt did in November 2012 to broker a truce between Israel and Hamas.
Kuwait requested an emergency Arab foreign ministers meeting to discuss “the deteriorating situation”, which a diplomat at the Arab League said will be held on Monday.
Israeli strikes on residential buildings in Gaza brought a rebuke from the UN’s human rights office over the number of civilian casualties.
“Even when a home is identified as being used for military purposes, any attack must be proportionate … and precautions must be taken to protect civilians,” said spokeswoman Ravina Shamdasani.
A group of 34 charities and NGOs also called for an end to the fighting.
“Military actions by all parties must stop,” said a statement signed by groups including ActionAid, CARE, Oxfam and Save the Children.
Amnesty International called for the United Nations “to immediately impose a comprehensive arms embargo on Israel, Hamas and Palestinian armed groups” and launch an inquiry into “violations committed on all sides”.
Since the start of Israel’s operation on Tuesday, about 525 rockets have struck the Jewish state, and Iron Dome has shot down around 138, an army spokeswoman told AFP on Saturday.
The army Twitter account said that overnight strikes “hit over 60 terror targets, making a total of 1,160 since the beginning of Operation Protective Edge.”
There were large protests outside the Israeli Embassy in Kensington High Street, west London on Friday night.