Israel will face accusations at the UN's top court on Thursday that it has committed "genocidal" acts in Gaza, charges the country's president dismissed as "atrocious" and "preposterous".

South Africa has lodged an urgent appeal to the International Court of Justice (ICJ) to force Israel to "immediately suspend" its military operations in Gaza.

Robed officials from both countries will go head-to-head in the Great Hall of Justice in the Peace Palace in The Hague, a world away from the devastation seen in Israel and Gaza since the October 7 Hamas attacks.

South Africa argues that Israel is breaking its commitments under the UN Genocide Convention, a treaty signed in 1948 as the world cried "never again" after the Holocaust.

As a fellow signatory to the treaty, South Africa can take Israel to the ICJ, which rules on disputes between countries and is often described as the "World Court".

South Africa has acknowledged the "particular weight of responsibility" of accusing Israel of genocide and "unequivocally" condemned the Hamas attacks that touched off the war in Gaza.

But in an 84-page submission to the court, Pretoria charged that Israel's bombing and invasion of Gaza is "intended to bring about the destruction of a substantial part of the Palestinian national, racial and ethnical group".

Israeli President Isaac Herzog lashed out at the accusations and laid out his country's likely defence.

"There's nothing more atrocious and preposterous than this claim," said Herzog.

"We will be in the International Court of Justice and we will present proudly our case of using self defence... under international humanitarian law," he said.

He said the Israeli army was "doing its utmost under extremely complicated circumstances on the ground to make sure that there will be no unintended consequences and no civilian casualties".

As it is an urgent procedure, the ICJ could rule in a matter of weeks.

Its rulings are final and cannot be appealed. However, countries do not always follow the court's verdicts – the ICJ has ordered Russia to stop its military operation in Ukraine for example. (AFP)