Israel pounded Gaza with air strikes on Friday, killing more than 15 people, including a top militant, and triggering a barrage of retaliatory rocket fire from the territory.
Israel said it had launched a pre-emptive strike against Islamic Jihad, killing a top commander of the Palestinian militant group it blames for a series of recent attacks inside Israel.
Islamic Jihad said the Israeli bombardment amounted to a "declaration of war", hours before it unleashed what it said was an "initial response" of 100 rockets towards Israel.
There were no immediate reports of casualties inside Israel, but in Gaza a child was among those killed, according to the territory's health ministry, which is run by the Islamist movement Hamas.
Hamas has fought four wars with Israel since seizing control of Gaza in 2007, most recently in May of last year. Islamic Jihad is a separate group, but aligned with Hamas.
The Israeli strikes were ongoing late on Friday, targeting what the army described as militant targets across the territory it has blockaded since the Hamas takeover.
Israeli Prime Minister Yair Lapid said the strikes were a "precise counter-terror operation against an immediate threat".
Flames poured out of a building in Gaza City following the first round of strikes, while wounded Palestinians were evacuated by medics.
Gaza's health ministry reported "a five-year-old girl, targeted by the Israeli occupation" was among nine people killed. A further 55 Palestinians were wounded, the ministry said.
Five-year-old Alaa Kaddum had a pink bow in her hair and a wound on her forehead, as her body was carried by her father at her funeral.
Islamic Jihad said several members of its military wing were among those killed, including "the great fighter Taysir al-Jabari 'Abu Mahmoud', the commander of the Al-Quds Brigade in the northern region of the Gaza Strip."
Hundreds of mourners gathered in Gaza City for the funerals of Jabari and others killed in the air strikes.
Israeli military spokesman Richard Hecht said "we are assuming about 15 killed in action" in Gaza, referring to Palestinian combatants. (AFP)