Israel is defending itself in the United Nations' top court starting Thursday against allegations that its ongoing military campaign in the neighboring Gaza Strip amounts to genocide of the Palestinian people -- a claim that Israel vehemently denies.

South Africa, which brought forward the allegations, is initially asking the Netherlands-based International Court of Justice to order an immediate suspension of the Israeli military offensive against Gaza's militant rulers, Hamas, as part of a landmark case that is likely to take years to resolve.

"Genocides are never declared in advance, but this court has the benefit of the past 13 weeks of evidence that shows incontrovertibly a pattern of conduct and related intention that justifies as a plausible claim of genocidal acts," South African attorney Adila Hassim told the panel of judges inside a packed courtroom in The Hague during Thursday's opening statements. "Nothing will stop the suffering except an order from this court."

South Africa insists Israel is committing genocide by design and that the country's latest war in Gaza is part of its decadeslong oppression of Palestinians. South Africa's ruling political party, the African National Congress, has a long history of solidarity with the Palestinian cause and sees parallels with its own struggle against the apartheid regime of white minority rule that ended in 1994.

"The violence and the destruction in Palestine and Israel did not begin on Oct. 7, 2023," South African Justice Minister Ronald Lamola said. "The Palestinians have experienced systematic oppression and violence for the last 76 years."

Israeli President Isaac Herzog has called South Africa's allegations “atrocious and preposterous,” while Secretary of State Antony Blinken has dismissed the case as "meritless."

Israeli Prime Minister Benjamin Netanyahu in response Thursday called South Africa's allegations "upside-down."

"Israel is fighting against murderous terrorists who have committed terrible crimes against humanity: they slaughtered, they raped, they burned, they dismembered, they killed children, women, the elderly, young men, young women. A terrorist organization that committed the most terrible crime against the Jewish people since the Holocaust, and now there are those who come to defend it in the name of the Holocaust," Netanyahu said. "We will continue to fight the terrorists, we will continue to repel the lies, we will continue to maintain our right to defend ourselves and secure our future."

National Security Council spokesman John Kirby called South Africa’s case "unfounded" and reiterated that "Israel has a right and responsibility to defend itself."

"We're going to continue to supply them with the tools and capabilities they need to do that," Kirby said Thursday.

Lawyers for Israel will address the court on Friday.

-ABC News' Joe Simonetti, Edward Szekeres and Morgan Winsor

The main highway connecting south and north Gaza, Salah al-Din Road, which Israeli forces used for a civilian corridor, has become impassable in Deir al Balah in central Gaza.

“When we came here, we were surprised -- Salah al-Din is a main road connecting the north and the south in four directions, 70 meters wide,” Gaza resident Yahya Deeb Al-Laham told ABC News. Now there’s “no infrastructure, no electricity, no roads, buildings and areas are non-existent … there is nothing here, there are no signs of life. Homes for families have completely disappeared and not a single one of them remains.”

The Israelis have recently left the area.

One of the families who followed Israeli military instructions, evacuating from northern Gaza to Deir al Balah, has been completely wiped out.

The surviving family member, Muhammad Fouad Abu Safi, returned to the site to sift through the rubble and try to find what might be left of his family.

“They left me no family member, no sister, no brother, no cousin, no child,” he told ABC News. “There were about 50 people here. Only three children, girls, came out alive … the rest here were taken out as body parts or decomposing bodies.”

“Humanity has ended, mercy has ended,” he said. “Neither from America nor from any country, there is no humanity or mercy.”

-ABC News’ Samy Zayara

Hamas leader Osama Hamdan said in a statement that the Israeli hostages held by Hamas in Gaza "will not return alive to their families" unless Israeli Prime Minister Benjamin Netanyahu and the Israeli leaders respond to Hamas’ conditions, "the first of which is a comprehensive and complete cessation of their aggression against the Gaza Strip."

A delegation from Israel is in Egypt for new discussions on swapping Israeli hostages held by Hamas in Gaza for Palestinians in prison in Israel, an Egyptian security source confirmed to ABC News.

About 577,000 people in Gaza, equal to a quarter of the population, are now starving, Arif Husain, chief economist for the U.N.'s World Food program, told ABC News.

Hussain has worked as an expert assessing hunger crises for 20 years and said, in terms of scale of severity and speed, he has never seen what is unfolding in Gaza right now, calling it “unprecedented.”

Even before the war with Israel, Gaza relied on humanitarian assistance to meet around 75% to 80% of its needs. With Israel now allowing very few supplies into Gaza, it has quickly run into massive shortages.

“If things continue as they are, or if things worsen, we are looking at a full fledged famine within the next six months," he said.

-ABC News' Patrick Reevell