Israel faced accusations at the World Court on Thursday of genocide in its war in Gaza, as the first residents returned to northern areas where Israeli forces have begun withdrawing, leaving behind scenes of total devastation.

Three months of Israeli bombardment has laid much of the narrow coastal enclave to waste, killing more than 23,000 people and driving nearly the entire population of 2.3 million Palestinians from their homes.

An Israeli blockade has sharply restricted supplies of food, fuel and medicine, creating what the United Nations describes as a humanitarian catastrophe.

Israel says its only choice to defend itself is by eradicating Hamas, the group that rules Gaza.

The case, brought by South Africa at the International Court of Justice (ICJ) in the Hague, accuses Israel of violating the 1948 genocide convention, enacted in the wake of the mass murder of Jews in the Holocaust, which mandates all countries to ensure such crimes are never repeated.

Israeli government spokesperson Eylon Levy compared the lawsuit to a centuries-old antisemitic conspiracy theory falsely accusing Jews of killing babies for rituals: "The State of Israel will appear before the International Court of Justice to dispel South Africa's absurd blood libel, as Pretoria gives political and legal cover to the Hamas rapist regime."

Amer Salah, 23, sheltering in a UN school in the Southern Gaza Strip after fleeing his home, told Reuters Gazans hoped the case would at last bring to bear international pressure forcing Israel to halt the war.

"Israel has always been a state above the law. They did what they did in Gaza because they knew they couldn’t be punished as long as America was on their side. It is time to change that,” he said.

"We salute South Africa, and we want the war to be stopped and the court can do that."

The preliminary hearings this week will consider whether the court should order Israel to stop fighting while it investigates the full merits of the case.

South African President Cyril Ramaphosa said his country was driven to bring the case by "the ongoing slaughter of the people of Gaza," motivated by South Africa's own history of apartheid.

The United States said Israel must do more to reduce civilian casualties, but called the genocide allegations "unfounded."

"In fact, it is those who are violently attacking Israel who continue to openly call for the annihilation of Israel and the mass murder of Jews," said State Department spokesperson Matt Miller.

Hamas official Sami Abu Zuhri told Reuters: "We urge the court to reject all pressure and take a decision to criminalize the Israeli occupation and stop the aggression on Gaza."