Syria boycotted a hearing at the United Nations' top court on Tuesday where the Netherlands and Canada accused Damascus of a years-long campaign of "institutionalized" torture against its own people.

The hearing was focused on a preliminary Dutch and Canadian request for the court to impose orders — known as provisional measures — on Syria to halt torture immediately to protect potential victims while their case accusing Damascus of breaching the torture convention proceeds through the International Court of Justice.

French President Emmanuel Macron Tuesday denounced "unacceptable blackmail" by Hamas after the Palestinian militant group threatened to execute some of the around 150 hostages it abducted in a weekend assault.

"The blackmail by Hamas after its terrorist acts is odious and unacceptable," Macron told reporters on a visit to Germany.

Over 180,000 Palestinians in the Gaza Strip are packed into United Nations shelters as Israeli warplanes pound the tiny territory of 2.3 million people after their Hamas militant rulers launched an unprecedented weekend attack on Israel.

Among them is 27-year-old Sabreen al-Attar. She sprang into action when she heard rocket after rocket whoosh over her farmland in Beit Lahiya just north of the Israeli border on Saturday. She knew from experience that Israeli retaliation would be swift and severe.

The latest Israel-Palestinian war reverberated around the world Tuesday, as foreign governments tried to determine how many of their citizens were dead, missing or in need of medical help or flights home.

Numerous countries also offered to play a role in mediating an end to the fighting, which already has killed at least 1,600 people. The death toll was expected to grow as Israel pummeled the Gaza Strip with airstrikes and sent Palestinians fleeing into U.N. shelters.

Hezbollah should not make the "wrong decision" of opening a second front against Israel as it battles attacks by Hamas, a senior U.S. defense official warned Monday.

"We are deeply concerned about Hezbollah making the wrong decision and choosing to open a second front to this conflict," the official told journalists.

Less than three weeks ago, Israeli Prime Minister Benjamin Netanyahu sat beside President Joe Biden and marveled that a "historic peace between Israel and Saudi Arabia" seemed within reach — a diplomatic advance that he predicted could lead to lasting peace between the Israelis and Palestinians.

Biden was equally optimistic, telling Netanyahu during their meeting in New York, "If you and I — 10 years ago — were talking about normalization with Saudi Arabia, I think we'd look at each other like, 'Who's been drinking what?'"

Israeli warplanes pounded downtown Gaza City, home to Hamas' centers of government, with relentless bombardments into early Tuesday, after Israel's prime minister vowed retaliation against the Islamic militant group that would "reverberate for generations."

The 4-day-old war has already claimed at least 1,600 lives, as Israel saw gun battles in the streets of its own towns for the first time in decades and neighborhoods in Gaza were reduced to rubble. Hamas also escalated the conflict, pledging to kill captured Israelis if strikes targeted civilians without warning.

A senior Hamas official has said only a small number of top commanders inside Gaza knew about the wide-ranging incursion launched into Israel, but that allies like Iran and Hezbollah "will join the battle if Gaza is subjected to a war of annihilation."

Ali Barakeh, a member of Hamas' exiled leadership, spoke to The Associated Press in his Beirut office as Israel bombarded Gaza and vowed a total blockade of the Hamas-ruled territory.

Egypt intensified its push Monday for diplomatic efforts to de-escalate spiralling violence in Gaza and Israel, his office said, after three days of fighting killed more than 1,200 people.

President Abdel Fattah al-Sisi, whose country has historically been a key mediator between Israel and the Palestinians, spoke with UAE Mohamed bin Zayed Al Nahyan, agreeing to "intensify discussions and diplomatic efforts to stop military escalation", Sisi's spokesman said.

Israeli air strikes on Gaza have killed four "prisoners" held by Hamas, said Monday the Palestinian militant group whose fighters carried out a surprise attack on Israel over the weekend.

"The occupation's bombing last night and today on the Gaza Strip led to the killing of four of the enemy's prisoners," the Ezzedine al-Qassam Brigades, the armed wing of Hamas, said on its website.