Israeli tanks pushed into southern and eastern districts of the Gazan city of Deir al-Balah for the first time on Monday, an area where Israeli sources said the military believes hostages may be held.

The area is packed with Palestinians displaced during more than 21 months of war in Gaza, hundreds of whom fled west or south after Israel issued an evacuation order, saying it sought to destroy infrastructure and capabilities of the militant group Hamas.

Tank shelling in the area hit houses and mosques, killing at least three Palestinians and wounding several others, local medics said.

"UN staff remain in Deir al-Balah, and two UN guesthouses have been struck, despite parties having been informed of the locations of UN premises, which are inviolable. These locations - as with all civilian sites - must be protected, regardless of evacuation orders," UN spokesperson Stephane Dujarric said.

The World Health Organization said its staff residence and main warehouse in Deir al-Balah was attacked on Monday.

Two WHO staff and two family members were detained by the Israeli military, said WHO Director-General Tedros Adhanom Ghebreyesus, adding that three were later released while one staff member remained in detention.

Israel's mission to the UN in New York declined to comment.

In its daily update, Gaza's Health Ministry said at least 130 Palestinians had been killed and more than 1,000 wounded by Israeli gunfire and military strikes across the territory in the past 24 hours, one of the highest such totals in recent weeks.

Israeli sources have said the reason the army had stayed out of the Deir al-Balah districts was because they suspected Hamas might be holding hostages there.

At least 20 of the remaining 50 hostages in captivity in Gaza are believed to be still alive. (Reuters)