Of the overall toll, eight were children and five were women.
By far the bloodiest strike was in northern Gaza City where a missile levelled a three-storey building, killing eight members of the Al-Dallu family, four of them children, and two other people, medics said.
Health ministry spokesman Ashraf al-Qudra named the dead as Mohammed al-Dallu, 35, a policeman, Suheila al-Dallu, 50, Samah al-Dallu, 22, and four children: Jamal and Sara, whose ages were not immediately available, Yussef, five, and 11-month-old Ibrahim al-Dallu.
The body of another woman, from the same family, was also pulled from the rubble but her identity was not immediately clear.
The other two victims, who lived next door, were named as Amina Mattar al-Muzzana, 83, and Abdullah Mohammed al-Muzzana, 22, he said.
The Israeli army had no immediate comment on the strike, admitting only that the air force had hit "a few targets in northern Gaza City."
Shortly afterwards, four more Palestinians were killed in three separate strikes, two in Gaza City and one on the Jabaliya refugee camp.
In Gaza City, Sami al-Ghafir was killed in a raid on the eastern Shejaiya neighbourhood, and Mohammed al-Awf was killed in the north of the city.
The strike on Jabaliya killed Suheil Hamada and his son Moamin as they were driving a water delivery truck through the refugee camp.
And in the early evening, a strike on Nuseirat refugee camp in central Gaza killed Aatiya Mubarak, a statement from the emergency services said.
Earlier strikes across the strip killed six more Palestinians, four of them children.
At around 2:00 am (0000 GMT) strikes on the northern town of Beit Hanun killed two toddlers, three-year-old Tamer Abu Saeyfan and his one-year-old sister Jumana.
Several hours later, 18-month-old Iyyad Abu Khusa was killed in a strike east of Bureij refugee camp in central Gaza. His two brothers, aged four and five, were critically wounded in the raid, Qudra said.
Medics later reported finding the body of woman under the rubble of a house in eastern Gaza City who had been killed in a strike earlier in the morning. They named her as Nawal Abdelaal, 52.
And in the late morning an air strike on a small house in the beachfront Shati refugee camp in Gaza City killed 13-year-old Tasneem al-Nahal and Ahmad al-Nahal, 25.
Both were from the same family but the relationship between them was not immediately clear.
The attacks came as diplomatic efforts intensified to broker an end to the bloodshed which began on Wednesday and has so far killed 67 Palestinians and injured more than 600.
Since the fighting began on Wednesday with an Israeli air strike which killed a top Hamas militant, three Israelis have been killed and more than 50 injured, a spokesman for the Magen David Adom emergency services told AFP.
The army said 10 of them were soldiers, one of whom was severely hurt.
On Sunday, nine people were injured by rockets in southern Israel, one of them seriously and two moderately.
So far, the military says it has struck more than 1,132 targets in the Gaza Strip, without saying how many strikes it had carried out.
In the same period, 544 rockets fired by Gaza militants have stuck southern Israel, and another 302 have been intercepted by the Iron Dome.