Ashraf al-Qedra of the Gaza health ministry said the 12 were killed and more than 10 others injured in the unending Israeli air strikes on the coastal enclave.
He said three people, including a child and an old woman, were killed in the town of Khan Younis.
"They were inside a car and their bodies were brought to the hospital," said al-Qedra, adding that the death toll since July 8 has climbed to 208 and more than 1,500 were injured.
Meanwhile, efforts on reaching a ceasefire between Israel and Palestinian militants were renewed Wednesday after reports said Palestine President Mahmoud Abbas headed to Egypt to discuss a truce in Gaza.
Abbas is to meet Hamas chief Khaled Meshaal and Hamas top leader Musa Abu Marzooq in Cairo to study an initiative presented by Egypt for a ceasefire.
Earlier, Israel reportedly told the residents of two neighbourhoods in Gaza city, Sheja'eya and Zaytoon, and the northern Gaza Strip town of Beit Lahia to leave their homes.
Israeli media reported that the army intended to destroy thousands of houses in the three areas because there were many tunnels dug underneath these houses.