The level three basement carpark in northern Israel's Rambam hospital is usually what it is called -- a place for people to leave their vehicles.

But in 30 hours this week, staff transformed the cavernous space into a huge underground hospital, equipping it with 1,300 beds, complete with fittings for oxygen, medical and sanitary supplies.

Israel is in the midst of a war to "crush" Hamas, after the Palestinian Islamists stormed its southern border at the weekend, shooting dead more than 1,200 people in the streets and in their homes.

Thousands of Palestinians fled Saturday to southern Gaza seeking refuge after Israel warned them to evacuate before an expected ground offensive against Hamas in retaliation for the deadliest attack in Israel's history.

Prime Minister Benjamin Netanyahu warned that nearly a week of fierce bombardment was "just the beginning" as Israel seeks to retaliate against Hamas after their fighters killed more than 1,300 a week ago.

The World Health Organization said Friday that a mass evacuation of the northern Gaza Strip would be "disastrous" for hospital patients, with hospitals in the south already at full capacity.

Israel has given Palestinians 24 hours to leave the northern Gaza Strip ahead of an expected ground offensive in retaliation against Hamas for the deadliest attack in Israeli history.

"The Palestinian Ministry of Health has informed WHO that it is impossible to evacuate vulnerable hospital patients without endangering their lives," the UN health agency said in a statement.

An Israeli embassy worker in Beijing was attacked on Friday, Israel's foreign ministry said.

Video geolocated by AFP shows the man was stabbed on a street in the Chinese capital.

"An Israeli employee of the Israeli embassy in Beijing was attacked today," a foreign ministry statement said, adding that the attack did not take place at the embassy compound.

"The employee is being treated at hospital and is in stable condition," the statement said.

"The motive for the assault is being looked into."

Thousands of protesters poured onto the streets of several Middle East capitals Friday in support of Palestinians amid Israeli air strikes on Gaza in reprisal for a surprise Hamas attack.

"No to the occupation! No to America!" chanted demonstrators gathered in central Baghdad after Shiite leader Moqtada Sadr called for a protest "in support of Gaza" and against Israel, an AFP journalist reported.

"This rally is aimed at condemning what is happening in occupied Palestine, the bloodletting and the violation of rights," said Abu Kayan, an organiser of the protest.

US Secretary of State Antony Blinken called Friday on Israel to limit Palestinian civilian deaths as he worked to set up safe areas in the Gaza Strip, seeking to keep calm in the Arab world following a major Hamas attack.

After a solidarity visit to Tel Aviv, Blinken, on a six-nation tour of the Arab world, again defended Israel's right to respond but also stepped up the tone on protecting innocent Palestinians.

"We have urged the Israelis to use every possible precaution to avoid harm to civilians," Blinken told a news conference in Qatar.

Hearing firsthand from Israeli survivors of the massive Hamas assault, US Secretary of State Antony Blinken experienced a rare scene for a diplomat: a cheering crowd.

During his 12-hour visit to Tel Aviv, Blinken declared, "The United States has Israel's back", a sentiment ordinary Israelis demonstrated was mutual, showing gratitude to their country's main ally.

Blinken's motorcade made a last-minute stop at the underground parking lot that has been turned into a makeshift donation hub after Israel's deadliest day in decades.

The United Nations on Thursday issued an emergency appeal for $294 million to address "the most urgent needs" in Gaza and the occupied West Bank, where more than 400,000 Palestinians have fled their homes in recent days.

The funds would be used to help more than 1.2 million people, the UN Office for the Coordination of Humanitarian Affairs (OCHA) said, adding that recent fighting in the region had left aid groups without adequate resources.

Yossi Landau has spent decades collecting corpses in Israel, but he almost reached breaking point recovering the remains of people killed by Gaza militants in the country's deadliest assault.

Landau woke to the sounds of sirens on Saturday, a moment he had become "used to" as Israelis took shelter from incoming rocket fire.

It was not until later that he realised the launches were "only a cover up, because the main part is the invasion" by Hamas militants, who swept across the Gaza border to kill an estimated 1,200 people.

Hundreds of Hollywood celebrities signed an open letter Thursday condemning the "barbaric acts" of Hamas fighters who murdered and abducted Israeli civilians in a shocking weekend assault.

"Wonder Woman" actress Gal Gadot, an Israeli, was among film and TV stars on the list, which also included Jamie Lee Curtis, Chris Pine, Mayim Bialik, Liev Schreiber, Michael Douglas and Jerry Seinfeld.

"Hamas murdered and kidnapped innocent men, women, and children. They kidnapped and murdered infants and the elderly," the open letter, released by Creative Community For Peace, said.