United Nations agencies and other aid organisations decried Sunday the devastating toll wreaked by six months of war in Gaza, warning that the situation is "beyond catastrophic".

"Six months is an awful milestone," the International Federation of Red Cross and Red Crescent Societies (IFRC) said, warning that "humanity has been all but abandoned".

The Gaza war broke out on October 7 with an unprecedented attack by Hamas militants that resulted in the deaths of 1,170 people, mostly civilians, Israeli figures show.

Hamas and Islamic Jihad militants also took more than 250 hostages, and 129 remain in Gaza, including 34 who the army says are dead.

Israel's retaliatory offensive has killed at least 33,175 people in Gaza, mostly women and children, according to the health ministry in the Hamas-run territory.

World Health Organization chief Tedros Adhanom Ghebreyesus reiterated his agency's condemnation of the "barbaric act of violence" that unleashed the war, and demanded "the release of remaining hostages".

But, he stressed on X, formerly Twitter, "this atrocity does not justify the horrific ongoing bombardment, siege and health system demolition by Israel in Gaza, killing, injuring and starving hundreds of thousands of civilians, including aid workers".

"The denial of basic needs -- food, fuel, sanitation, shelter, security and healthcare -- is inhumane and intolerable."

Of Gaza's 36 main hospitals, only 10 remain even partially functional, according to the WHO.

 'Stain on all of humanity' 

 Tedros voiced particular outrage at "the deaths and grievous injuries of thousands of children in Gaza", which he said would "remain a stain on all of humanity".

"This assault on present and future generations must end."

UNICEF chief Catherine Russell pointed out that more than 13,000 children were reportedly among those killed.

"Homes, schools, and hospitals in ruin. Teachers, doctors, and humanitarians were killed. Famine is imminent," she said on X on Saturday.

"The level and speed of destruction are shocking. Children need a ceasefire NOW."

The UN humanitarian chief Martin Griffiths insisted Saturday that there needed to be "a reckoning for this betrayal of humanity".

The IFRC chief Jagan Chapagain meanwhile described the situation as "beyond catastrophic", with "millions of lives at risk of hunger."

"An urgent and unhindered flow of humanitarian aid must be ensured to reach those in need. Not tomorrow, but now."

IFRC said 18 members of its network -- 15 staff and volunteers with the Palestinian Red Crescent Society (PRCS) and three from Magen David Adom (MDA) -- had been killed since October 7.

"These deaths are devastating and unacceptable," Chapagain said on X.

For the International Committee of the Red Cross (ICRC), the caretaker of the Geneva Conventions, "a steady flow of humanitarian aid" into Gaza was vital, but it was "only part of the solution".

"Both sides must conduct their military operations in a way that spares civilians caught in the middle," it said on X.

Tedros pointed out that over 70 percent of those who have died in Gaza have been women and children, insisting this was "a compelling reason to halt the war".

"We urge all parties to silence their guns. We appeal for peace. Now."