Israel launched pre-dawn air strikes on Tuesday in Gaza, according to the health ministry in the Hamas-controlled Palestinian territory.
The ministry said that women and children were among the 12 dead but did not give further details as to the identity of the victims.
The Israeli army said it had targeted three leaders of Islamic Jihad, which it considers a terrorist organisation, and had hit "weapon manufacturing sites" belonging to the group.
Islamic Jihad confirmed three senior officials were killed, naming them in a statement as Jihad Ghannam, secretary of the Al-Quds Brigades military council, and Khalil al-Bahtini, also of the council and commander of the military wing in northern Gaza.
The third, Tareq Ezzedine, was described by Islamic Jihad as "one of the heads of military action" in the occupied West Bank who operated from Gaza.