Emergency services initially said three people were killed and five wounded, but later announced that two people had died of their injuries after the raid in Al-Zawayda near the Nusseirat refugee camp.
The air raid hit a family home, witnesses and medics said.
Doctors at the Al-Aqsa Hospital in Deir al-Balah said the five dead all came from the same family — a 28-year-old father, his 26-year-old wife and their two boys aged three and four.
The father’s 45-year-old aunt was also killed, they said.
The Israeli military said it had carried out around 20 air strikes over the Gaza Strip early on Saturday and that three rockets or mortar rounds had hit southern Israel near the border with Gaza.
Hostilities in the six-week war resumed on Tuesday as Egyptian-brokered truce talks collapsed.
Emergency services say 81 people have died in the Palestinian enclave since then. On Friday, an Israeli child was killed and seven other people wounded, one of them critically, by mortar and rocket fire from Gaza.
At least 2,097 Palestinians have been killed since July 8, 70 percent of them civilians, according to the United Nations.
There have also been 68 people killed on the Israeli side, all but four of them soldiers.