The Israeli military said on Saturday it had seen a "significant movement" south of Palestinian civilians, a day after ordering Gaza City residents to flee, as Israel's prime minister promised further retaliation for Hamas' rampage last week.

U.S. President Joe Biden said consultations were under way with regional governments on the humanitarian crisis in Gaza as trapped Palestinians endured a power blackout and shortages of food and water amid fierce Israeli bombing.

Israel has vowed to annihilate Hamas for the attack a week ago, in which its fighters killed 1,300 Israelis, mainly civilians, and seized scores of hostages.

Israel has since put the Hamas-run Gaza Strip, home to 2.3 million Palestinians, under a total siege and bombarded it with unprecedented air strikes. Gaza authorities say 1,900 people have died.

On Friday, more than a million residents of northern Gaza had received a notice from Israel to flee south within 24 hours, a deadline that expired at 5 a.m. (0200 GMT).

"We have seen a significant movement of Palestinian civilians towards the south," Israeli military spokesperson Lieutenant Colonel Jonathan Conricus told a video briefing early on Saturday.

He did not mention the deadline and did not take questions.

"Around the Gaza Strip, Israeli reserve soldiers in formation (are) getting ready for the next stage of operations," he added.

"They are all around the Gaza Strip, in the south, in the centre and in the north, and they are preparing themselves for whatever target they get, whatever task.

"The end state of this war is that we will dismantle Hamas and its military capability and fundamentally change the situation so that Hamas never again has the ability to inflict any damage on Israeli civilians or soldiers."

Hamas vowed to fight to the last drop of blood and told residents to stay.

The Israeli military said on Friday tank-backed troops had mounted raids to hit Palestinian rocket crews and gather information on the location of hostages, the first official account of ground troops in Gaza since the crisis began.

An Israeli drone killed a number of militants who tried to infiltrate Israel from Lebanon, it added on Saturday.

"We are striking our enemies with unprecedented might," Israeli Prime Minister Benjamin Netanyahu said in a rare statement televised after the Jewish Sabbath began on Friday. "I emphasise that this is only the beginning."

Tens of thousands of Palestinians were estimated to have headed south from northern Gaza after the Israeli order, according to the United Nations, which said more than 400,000 Palestinians had been internally displaced due to hostilities before the directive.

Many others, however, said they would stay. "Death is better than leaving," said Mohammad, 20, outside a building smashed by an Israeli air strike near the centre of Gaza.

Mosques broadcast the message, "Hold on to your homes. Hold on to your land."

The United Nations and other organisations warned of a disaster if so many people were forced to flee, calling for the siege to be lifted to let in aid.

"We need immediate humanitarian access throughout Gaza, so that we can get fuel, food and water to everyone in need. Even wars have rules," U.N. Secretary General Antonio Guterres said on Friday.

Biden, in a speech at a Philadelphia shipping terminal, said tackling the humanitarian crisis was a top priority. U.S. teams in the region were working with Israel, Egypt, Jordan, other Arab governments and the United Nations, he said.

"The overwhelming majority of Palestinians had nothing to do with Hamas and Hamas' appalling attacks," Biden said. "And they're suffering as a result as well."

U.N. spokesperson Stephane Dujarric said it was impossible for Gazans to heed Israel's order to move south without "devastating humanitarian consequences", prompting a rebuke from Israel that the U.N. should condemn Hamas and support Israel's right to self-defence.

"The noose around the civilian population in Gaza is tightening," U.N. aid chief Martin Griffiths wrote on social media. "How are 1.1 million people supposed to move across a densely populated war zone in less than 24 hours?"

Israel said it would allow the safe movement of Gazans on the two main roads south in the territory between 10 a.m. and 4 p.m.

'Distance yourself from Hamas'

Mahmoud Abbas, president of Hamas rival Palestinian Authority, told U.S. Secretary of State Antony Blinken in Jordan the forced displacement would constitute a repeat of 1948, when hundreds of thousands of Palestinians fled or were driven from what is now Israel.

Most Gazans are descendants of such refugees.

Gaza is one of the most crowded places on earth, and for now there is no way out. In addition to Israel's blockade, Egypt has resisted calls to open its border with Gaza.

U.S. Defence Secretary Lloyd Austin met Israeli Defence Minister Yoav Gallant on Friday. Austin said military aid was flowing into Israel but the time was one for resolve, not revenge.

"The path will be long, but ultimately I promise you we will win," Gallant said.

Blinken met King Abdullah in Jordan as well as Abbas, whose Palestinian Authority exercises limited self-rule in the Israeli-occupied West Bank but lost control of Gaza to Hamas in 2007. Blinken later flew to Qatar, a U.S. ally with influence among Islamist groups.

In the West Bank, demonstrators supporting Gaza fought gun battles with Israeli security forces. Palestinian officials said 16 people were shot dead.

There have also been fears of hostilities spreading, including to Israel's northern border with Lebanon, where clashes this week have already been the deadliest since 2006.

Reuters video journalist Issam Abdallah was killed on Friday while working in southern Lebanon by missiles fired from the direction of Israel, according to another Reuters videographer on the scene. Six other journalists were injured.

Lebanese Prime Minister Najob Mikati and a Hezbollah lawmaker blamed the incident on Israel. Israel's U.N. envoy said it would investigate what had happened in the area.

"We always try to mitigate and avoid civilian casualties," said the envoy, Gilad Erdan. "Obviously, we would never want to hit or kill or shoot any journalist that is doing its job."