People gather to identify the bodies of family members killed in Israeli strikes on Rafah in the southern Gaza Strip, at the al-Najjar hospital on December 1, 2023. AP
Speaking on the sidelines of the UN's COP28 climate talks in Dubai, Robert Mardini said "people are at a breaking point, hospitals are at a breaking point, the whole Gaza Strip is in a very precarious state".
Resumption of fighting brings the people of Gaza "back to the nightmarish situation they were in before the truce took place", said Mardini, director-general of the International Committee of the Red Cross.
He noted their "suffering, destruction, fear, anxiety and precarious living conditions".
Israel's military said fighter jets were striking Gaza on Friday, as AFP journalists reported air attacks in the north and south of the territory.
"There is nowhere safe to go for civilians," Mardini said, stressing the challenges hospitals and humanitarian organizations are facing.
"We have seen in the hospitals where our teams have been working, that over the past days, hundreds of severely injured people have arrived," he said.
"The influx of severely wounded outpaced the real capacity of hospitals to absorb and treat the wounded, so there is a massive challenge."
"We have seen so far that releases of captives only happened when there was a truce because you need certain conditions to be met to do this," Mardini said.
"We stand ready as the ICRC to facilitate those releases."
Renewed fighting also threatens the entry of aid to Gaza where about 80 percent of the population is displaced and grappling with shortages of food, water and other essentials.
"With the resumption of hostilities, the likelihood will be that less aid will get in," Mardini said.
"More importantly, humanitarian organizations, like the Palestinian Red Crescent Society, and others such as the ICRC and UN agencies, will have reduced capacities to deliver aid to the people," he added.
"Even people will have reduced capacities to get to places where they could receive aid."