The emergency department at the Al-Shifa hospital in northern Gaza, devastated by Israeli bombardments, is "a blood bath" and is "in need of resuscitation", the WHO said Sunday.
A team from the World Health Organisation and other United Nations agencies was able to deliver medical supplies Saturday to the hospital, the largest in the Palestinian territory.
In a statement the WHO said that "tens of thousands of displaced people are using the hospital building and grounds for shelter", and that there is "a severe shortage" of drinking water and food.
"The team described the emergency department as a 'bloodbath', with hundreds of injured patients inside, and new patients arriving every minute," the organisation said, adding that "patients with trauma injuries were being sutured on the floor... (and) no pain management is available".
The hospital is functioning at a minimal scale with very few staff and the WHO said "critical patients are being transferred to Al-Ahli Arab Hospital for surgeries".
Operating theatres are not working due to a lack of oxygen and supplies, and according to the WHO team, it is a "hospital in need of resuscitation".
Only 30 patients can receive dialysis daily.
All health infrastructure in the Gaza strip has been hard hit by bombardments and ground operations carried out by the Israeli army since the unprecedented October 7 attack by Hamas on Israeli territory.
The attack saw about 1,140 people killed, mostly civilians, and 240 hostages taken, according to the Israeli authorities' latest figures.
According to Hamas, Israel's retaliatory offensive in Gaza has killed 18,800 people, mostly women and children.
Israel has accused Hamas of running a command centre below the hospital -- which has special protection under the laws of war -- despite repeated denials from hospital officials and the Palestinian fighter group.
The WHO said it would reinforce Al-Shifa "in the coming weeks" in order for it to resume basic services.
"Up to 20 operating theatres in the hospital, as well as post-operative care services, can be activated if provided with regular supplies of fuel, oxygen, medicines, food, and water," the WHO said, along with the necessary staff.
Currently, Al-Ahli Arab is the only hospital "partially functioning" in the entire north of the Gaza Strip, while three hospitals -- Al-Shifa, Al Awda and Al Sahaba Medical Complex -- are functioning at a minimum.
Before the war, there were 24 operational facilities.
The WHO has also expressed concerns about the Kamal Adwan hospital, where the Hamas health ministry said on December 13 that the Israeli army had fired on patient rooms in the besieged facility.