The Gaza civil defence agency said at least 48 people were killed Thursday by Israeli strikes in the Palestinian territory, including several killed in an air strike in Gaza City ahead of a major offensive to capture the area.

According to civil defence spokesman Mahmud Bassal, at least eight people were killed by an air strike in the Sabra neighbourhood in Gaza City.

Sabra and the nearby Zeitoun district, have been hit hard in recent days by shelling and air strikes, according to residents.

The civil defence agency also provided details for strikes on several areas across the strip, including the al-Manasra camp in central Gaza, an aid centre in Khan Yunis in the south of the territory, and other points in the north around Gaza City.

When contacted by AFP, the Israeli military asked for coordinates and specific timeframes to comment on the reports.

An AFP journalist at Gaza City's Al-Shifa Hospital saw several bodies brought to the grounds of the facility after being killed by Israeli strikes.

Israel hammered Gaza City and its outskirts overnight, residents said Thursday, as the military announced it had taken the initial steps in its push to capture Hamas's last major stronghold.

The newly approved plan authorises the call-up of roughly 60,000 reservists, deepening fears the campaign will worsen the already catastrophic humanitarian crisis in the Gaza Strip.

"We are advancing with the efforts toward operations in Gaza City," military chief Eyal Zamir told troops Thursday.

"We already have troops operating on the outskirts of the city, and more forces will join them later on."

Israel's plans to expand the fighting and seize Gaza City have sparked international outcry as well as domestic opposition, with the Red Cross joining the condemnation Thursday, calling the moves "intolerable".

Ahead of the offensive, the Israeli military said the call-up of the reservists would begin in early September, adding the second phase of operation "Gideon's Chariots" had begun.

Gaza City residents described relentless bombardments overnight.

"The house shakes with us all night long -- the sound of explosions, artillery, warplanes, ambulances, and cries for help is killing us," Ahmad al-Shanti told AFP.

"The sound is getting closer, but where would we go?"

Another resident, Amal Abdel-Aal, said she watched the heavy strikes on the area, a week after being displaced from her home in Gaza City's Al-Sabra neighbourhood.

"No one in Gaza has slept — not last night, not for a week. The artillery and air strikes in the east never stop. The sky flashes all night long," she added.

Gaza civil defence agency spokesman Mahmud Bassal said air strikes and artillery fire overnight targeted areas to the northwest and southeast of the city.

A group of AFP journalists near Israel's border with Gaza also witnessed an air strike by a fighter jet on the northern outskirts of Gaza City Thursday afternoon, with a massive explosion followed by a large plume of smoke rising into the sky.

Several sporadic explosions were heard afterward but it was not possible to determine their origin.

Strikes increased in pace later in the afternoon with several large explosions heard near the border.

The UN humanitarian agency has warned the Israeli plan to expand military operations in Gaza City would have "a horrific humanitarian impact" on the already exhausted population.

"Forcing hundreds of thousands to move south is a recipe for further disaster and could amount to forcible transfer," OCHA said.

Others reportedly "remain trapped, completely cut off from food, water and medicine supplies", it added.

The Israeli military said this week it had also begun informing medical personnel and aid groups in northern Gaza to start making evacuation plans and transferring their equipment to the south.

The Gaza health ministry, however, rejected that call Thursday, saying it would not agree to "any step that would undermine what remains of the health system after the systematic destruction carried out by the occupation authorities".

As Israel tightened its grip on Gaza City's outskirts, mediators were still waiting for an official Israeli reaction to their latest ceasefire proposal, which Hamas accepted earlier this week.

Israel and Hamas have held indirect negotiations throughout the nearly two-year conflict, paving the way for a pair of short ceasefires during which Israeli hostages were freed in exchange for Palestinian prisoners.

Sources from Hamas and its ally Islamic Jihad told AFP this week that the latest ceasefire proposal calls for the release of 10 hostages and 18 bodies from Gaza. The remaining hostages would be released in a second phase alongside talks for a wider settlement.

Israel, however, has insisted that any agreement involve freeing all the remaining hostages at once.

Israel's offensive has killed at least 62,122 Palestinians, most of them civilians, according to figures from the health ministry in Hamas-run Gaza, which the United Nations considers reliable.