As fighting intensifies around the hospital in Khan Younis, hundreds of patients and health workers have fled, said Tedros Adhanom Ghebreyesus.
"Currently 350 patients and 5000 displaced people remain at the hospital. The hospital is running out of fuel, food and supplies," he wrote on X, with an accompanying video from the hospital.
He said fighting continues in the vicinity of the hospital and noted access for resupplying the "remains challenging."
"We appeal for a an immediate ceasefire, so that we can replenish urgently needed lifesaving supplies," he said.
Israel has launched a deadly offensive on the Gaza Strip since an Oct. 7 attack by Hamas, killing at least 26,083 Palestinians and injuring 64,487 others. Nearly 1,200 Israelis are believed to have been killed in the Hamas attack.
The Israeli war has left 85% of Gaza’s population internally displaced amid acute shortages of food, clean water and medicine, while more than half of the enclave’s infrastructure has been damaged or destroyed, according to the UN.