Israel's military warned on Saturday that deadly air strikes against Palestinian militants in Gaza could last a week, as cross-border fire reverberated for a second day in the worst escalation since last year's war.

Israel has said it was necessary 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 since Friday, adding that more than 140 have been wounded.

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.

Egyptian President Abdel Fattah al-Sisi said Cairo is talking "around the clock" with both sides, which so far have rejected calls for hostilities to end.

Daily life in Gaza 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.

Gaza's health ministry said the next few hours would be "crucial and difficult", warning it risked suspending vital services within 72 hours as a result of the lack of electricity.

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

Islamic Jihad's leader in Gaza, Mohammed al-Hindi, said: "The battle is still at its beginning."

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

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. (AFP)