Israel has said it was forced to launch a "pre-emptive" operation against Islamic Jihad, saying the group was planning an imminent attack following days of tensions along the border with Gaza.

Health authorities in the Palestinian enclave, which is controlled by the Islamist group Hamas, said a five-year-old girl was among 15 people killed in the Israeli bombardment, adding that more than 120 others have been wounded.

In Jabalia, in the north of Gaza, Fouad Farajallah was on Saturday surveying what remained of his home, hit by an Israeli strike the day before.

"I was sitting here on the sofa, with my wife and children, and suddenly it all fell on us," he said, surrounded by debris from a partially collapsed roof in a strike he said wounded his wife and son.

Israel's ongoing strikes are being met with barrages of rockets from the Palestinian side, stoking fears of a repeat of an 11-day conflict that devastated Gaza in May 2021.

Daily life in the enclave has come to a standstill, while the electricity distributor said the sole power station shut down due to a lack of fuel after Israel closed its border crossings.

Civilians on the Israeli side meanwhile took refuge in air raid shelters, but there were no immediate reports of casualties.

In Kibbutz Nahal Oz, an Israeli community beside the Gaza border, resident Nadav Peretz said he has been "in the bomb shelter or around it" since Friday.

"We recognise that on the other side too there is an uninvolved civilian population, and on both sides children deserve to enjoy their summer vacation," the 40-year-old said.

- 'Declaration of war' -

An Israeli military spokesman said its forces were "preparing for the operation to last a week," and told AFP that the army is "not currently holding ceasefire negotiations".

Sources within Islamic Jihad ruled out a ceasefire soon, with one saying: "for the movement, the focus is on the battlefield".

Israel and Islamic Jihad confirmed the killing of Taysir al-Jabari, a key commander of the militant group, in a Friday strike.

The Israeli bombardment has been condemned by Islamic Jihad's top backer Iran.

Israel has "once again showed its occupying and aggressive nature to the world," said a statement from President Ebrahim Raisi's office.

Jamal al-Fadi, a political science professor at Gaza's Al-Azhar University, said he expected the violence to end "within days".

"Islamic Jihad is reacting in a limited manner and by doing so is preventing the occupation (Israeli military) from intensifying its air strikes," he told AFP.

Islamic Jihad is aligned with Hamas, but often acts independently. Both are blacklisted as terrorist organisations by much of the West.

Hamas has fought four wars with Israel since seizing control of Gaza in 2007, including the conflict last May.

A flare-up with Islamic Jihad came in 2019, following Israel's killing of Baha Abu al-Ata, Jabari's predecessor. Hamas did not join the fray in that conflict.

Hamas's moves now could prove crucial.

Mairav Zonszein, a senior analyst at the International Crisis Group think tank, said Hamas currently has an interest in staying on the sidelines as it has recently won more economic aid for Gazans.

"But if more civilians are killed, then it will feel obligated to respond," she told AFP.

- Five-year-old girl -

Mohammed Abu Salameh, the director of Shifa, Gaza city's main hospital, said medics are facing "acute shortages of medical supplies".

On Friday, the health ministry reported "a five-year-old girl, targeted by the Israeli occupation" was among those killed.

The girl, 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.

Israeli military spokesman Richard Hecht said Friday "we are assuming about 15 killed in action" in Gaza, referring to Palestinian combatants.

The Gaza strikes followed the arrest in the occupied West Bank of two senior members of Islamic Jihad, including Bassem al-Saadi, who Israel accuses of orchestrating recent attacks.

Israel on Saturday broadened its operation against Islamic Jihad, announcing the arrest of 19 people in the West Bank it said were members of the group.

Israel has conducted a wave of often deadly raids inside West Bank towns and cities since mid-March in response to lethal attacks on Israelis.