Israel's intensified attacks on residential areas in Gaza are causing a high number of casualties, UN humanitarians said Thursday.

The UN Office for the Coordination of Humanitarian Affairs (OCHA) said that airstrikes and shelling have intensified in parts of Gaza City over the past couple of days, as attacks also continue in Deir al-Balah and Khan Younis.

"If the announced Israeli ground operation in Gaza City moves ahead, thousands of families already in appalling humanitarian conditions could be pushed over the edge even further," OCHA said. "With 86 percent of the Strip now in Israeli-militarized zones or under displacement orders, aid groups lack the access and supplies to address the dire needs at the scale required."

The office added that in recent days, temperatures have soared, and hundreds of thousands of people are now left without protection from the heat.

"Nearly everyone in Gaza has been displaced at least once since the war began, and the makeshift shelters they managed to improvise or acquire have often either worn out or been abandoned in the rush to flee," OCHA noted.

"We reiterate our calls for a ceasefire, the unconditional and immediate release of all hostages and the unimpeded humanitarian access," the office said.