At least 500 people have been killed in a strike on Al Ahli Arab Hospital in the middle of Gaza City, according to the Gaza Health Ministry.

The hospital served as a shelter for thousands of residents who fled their homes in northern Gaza, seeking safety from Israeli airstrikes. Residents stranded in Gaza are running out of medicine, food, water and power.

The Israel Defense Forces denied responsibility for the attack, saying a failed launch by the Palestinian Islamic Jihad caused the blast.

"From the analysis of the operational systems of the IDF, an enemy rocket barrage was carried out towards Israel, which passed in the vicinity of the hospital, when it was hit," IDF said.

The World Health Organization condemned the attack on the hospital and called for "the immediate active protection of civilians and health care" workers.

This hospital "was one of 20 in the north of the Gaza Strip facing evacuation orders from the Israeli military," the WHO said in a statement. "The order for evacuation has been impossible to carry out given the current insecurity, critical condition of many patients, and lack of ambulances, staff, health system bed capacity, and alternative shelter for those displaced."

The United Nations Relief and Works Agency said earlier Tuesday that Gaza hospitals were estimated to have less than 24 hours of fuel left.

Palestinian Authority President Mahmoud Abbas has declared three days of mourning following the attack.