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

Health authorities in Gaza, a Palestinian enclave controlled by the Islamist group Hamas, said a five-year-old girl was among 12 people killed by the Israeli bombardment. More than 80 others have been wounded.

The Israeli military warned Saturday it was "preparing for the operation to last a week," while the sole power station in Gaza ground to a halt due to a lack of fuel after Israel closed border crossings.

Israel and Islamic Jihad have both confirmed the killing of Taysir al-Jabari, a key leader of Islamic Jihad, in a Friday strike on a building in the west of Gaza city.

Islamic Jihad is aligned with Hamas, but often acts independently. It said the initial Israeli bombardment amounted to a "declaration of war" and unleashed a barrage of rockets towards Israel.

Sources within the militant group ruled out a ceasefire soon, with one saying: "for the movement the focus is on the battlefield," while an Israeli military spokesman told AFP that the army is "not currently holding ceasefire negotiations".

The rocket fire and Israeli strikes were continuing on Saturday, risking a repeat of an 11-day conflict in May 2021 that devastated Gaza and forced countless Israelis to rush to bomb shelters.

Daily life in the Palestinian enclave has come to a standstill, with streets largely deserted and most shops closed.

- 'Will not shy away' -

Israel's closure of a key goods crossing four days ago resulted in Gaza's power plant shutting down on Saturday, due to a "fuel shortage," a spokesman for the enclave's electricity distributor said.

Diesel for the plant is usually trucked in from Egypt or Israel, which closed its two border crossings with Gaza on Tuesday citing security concerns.

"Israel isn't interested in a wider conflict in Gaza, but will not shy away from one either," Prime Minister Yair Lapid said in a nationally televised address on Friday.

Air raid sirens sounded across southern Israel on Saturday, but there were no immediate reports of casualties or major damage.

Officials in border areas urged people to stay close to shelters, which have also been opened in the coastal city of Tel Aviv.

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

Both Hamas and Islamic Jihad are considered terrorist organisations by much of the West.

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 episode.

Hamas's moves now could prove crucial, with the group facing pressure from some to restore calm in order to improve economic conditions in Gaza.

- 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.

Israeli tanks were lined up along the border, after the military said Thursday it was reinforcing its troops.

The measures 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 it said were members of the group in the occupied West Bank.

Israel has conducted a near relentless wave of often deadly raids inside the West Bank since mid-March in response to lethal attacks on Israeli citizens.