The Israeli strikes come hours after Islamist group Hamas said it had brokered a deal for Gaza's militant groups to stop firing on Israel.
On Thursday, members of its military wing hit an Israeli school bus with an anti-tank shell, injuring two people.
Friday's deaths raise to 10 the number of Palestinians killed since Thursday, amid the worst fighting since 2009.
Last night, Hamas - which governs the Gaza Strip - met with other militant factions and agreed to enforce a ceasefire if Israel also stopped firing.
However neither side seems to have stopped for long, says the BBC's Jon Donnison in Gaza City.Israel has carried out air strikes on dozens of targets in the Gaza Strip
Israeli artillery fire and air strikes have been heard across Gaza throughout much of the day, our correspondent says, and at least six rockets were fired into Israel, hitting an area north of the Gaza Strip, according to Israeli police.
It is not clear whether the fresh violence signals an end to the truce, or whether the firing of rockets was carried out by a Palestinian splinter group that had not signed up to the ceasefire.
Israel says it is responding to the school bus attack on Thursday near the Nahal Oz kibbutz. A 16-year-old boy suffered a serious head wound and was taken to hospital for surgery.
Militants from the al-Qassam Brigades, the military wing of Hamas, hit the bus with an anti-tank shell. They said it was in response to the killing of Hamas leaders last week.
The bus attack was condemned by the US, which said it was particularly concerned by reports that the Gaza militants had used an advanced anti-tank weapon to target civilians.
At least 10 Palestinians have been killed since Thursday and more than 40 have been injured.
Several of those killed were civilians, including a mother and her 21-year-old daughter, as well as a 55-year-old man in the southern city of Khan Younis, Palestinian medics say.
Another 50-year-old civilian who had been sitting outside his home died when he was hit by tank fire on Thursday, an official in Gaza said.
Four others were identified as leaders of the Qassam Brigades and two of Friday's victims had been members of the group, Hamas said.
Last month saw some of the worst violence since Israel launched a major offensive in Gaza in December 2008.
In one week in March, at least 10 Palestinians - including several civilians and children - were killed by Israeli attacks.
In the same period, militants in Gaza fired more than 80 rockets and mortar shells into southern Israel.
Israel says it holds Hamas responsible for all attacks coming out of Palestinian territory, even if it is other militant groups carrying them out.
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