The strike at Al-Aqsa hospital was witnessed by a World Health Organization team sent there to assess needs and to collect incubators for the north of Gaza, Tedros said in a post on X, formerly Twitter.

Israel's military denied that the hospital was damaged.

"A WHO team was on a humanitarian mission at Al-Aqsa Hospital in Gaza, when a tent camp inside the hospital compound was hit by an Israeli airstrike today," the WHO chief wrote.

He gave no details of the victims, but added: "WHO staff are all accounted for."

The Israeli army said on X that one of its planes "struck an operational Islamic Jihad command center and terrorists positioned in the courtyard of the Al-Aqsa Hospital in the area of Deir al Balah.

"Following the precise strike, the Al-Aqsa hospital building was not damaged and its function was not affected," it added.

Tedros said in a later post that 21 patients had died at Al-Shifa hospital in the northern Gaza City since March 18.

The hospital, which was Gaza's largest before the war broke out, has only one bottle of water available for every 15 people, he said.

Tedros renewed his appeal for patients, health professionals and aid workers to be protected during the conflict, and for attacks on hospitals to stop.

"We again call for protection of patients, health personnel and humanitarian missions," he added.

"The ongoing attacks and militarisation of hospitals must stop. International humanitarian law must be respected."

Tedros urged all parties to the conflict to comply with the UN Security Council resolution calling for a ceasefire.

Since an unprecedented October 7 attack Hamas launched out of the Gaza Strip, Israel has said it will destroy the movement and has relentlessly bombed the Palestinian territory.

There are 10 hospitals in the Gaza Strip still functioning at a minimum level, compared to 36 before the start of the war, according to the WHO.

As well as air strikes there has also been fierce ground fighting in and around hospitals in the territory, where many Gazans fleeing the violence have taken shelter.

Hamas's October 7 attack resulted in about 1,160 deaths in Israel, mostly civilians, according to an AFP tally of Israeli official figures.

Israel's retaliatory campaign has killed at least 32,782 people, mostly women and children, according to the Hamas-run health ministry in Gaza.