Israel says its infantry and tanks have carried out raids inside the Gaza Strip, its first announcement of a shift from an air war to ground operations to root out a week after their deadly rampage in southern Israel.
Some Gaza residents were abandoning homes on Friday to escape from the path of an Israeli onslaught, after Israel ordered more than a million people to leave the northern half of the Gaza Strip within 24 hours.
Hamas told them not to go.
Israeli military spokesman Rear Admiral Daniel Hagari said troops backed by tanks had mounted raids to attack Palestinian rocket crews and seek information on the location of hostages, the first official account of ground troops in Gaza since the crisis began.
Several thousand residents could be seen on roads heading out of the northern part of the Gaza Strip but it was impossible to assess their numbers.
Many others said they would not go.
Hamas, which controls the densely populated Palestinian territory, vowed to fight until the last drop of blood.
The Israeli military said a significant number of Gazans had begun moving southwards "to save themselves".
"Death is better than leaving," said Mohammad, 20, standing in the street outside a building reduced to rubble in an earlier Israeli air strike near the centre of Gaza.
"I was born here, and I will die here. Leaving is a stigma."
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.
"The United Nations considers it impossible for such a movement to take place without devastating humanitarian consequences," UN spokesman Stephane Dujarric said, prompting a rebuke from Israel that the UN should condemn Hamas and support Israel's right to self-defence.
The International Committee of the Red Cross said it would be impossible for aid organisations to help while Gaza is under Israeli siege: "The needs are staggering, and humanitarian organisations must be able to increase aid operations."
White House national security spokesman John Kirby said such a huge evacuation was a "tall order" but that the US would not second guess Israel's decision to tell civilians to get out.
"We understand what they're trying to do and why they're trying to do this — to try to isolate the civilian population from Hamas, which is their real target," he said on MSNBC.
Israel's evacuation order applies to the northern half of the Gaza Strip, including the enclave's biggest settlement Gaza City.
The UN said it had been told that Israel wanted the area's entire population - about half the 2.3 million Gazans - to move across the Gaza Wadi wetland that bisects the enclave.
Mahmoud Abbas, president of the Palestinian Authority that is a rival of Hamas, told US Secretary of State Antony Blinken in Jordan that the forced displacement of Palestinians in Gaza would constitute a repeat of 1948, when hundreds of thousands of Palestinians fled or were driven from what is now Israel.
Most Gazans are the descendants of such refugees.
Gaza is already one of the most crowded places on earth, and for now there is no way out.
Israel has imposed a total blockade and Egypt, which also has a border with the enclave, has so far resisted calls to open it to fleeing residents.
Since Hamas fighters burst across the barrier fence and killed 1,300 Israelis on Saturday, Israel has responded with the most intensive air strikes of its 75-year conflict with the Palestinians.
Gaza authorities say 1,800 people have been killed; the United Nations says 400,000 people have already been made homeless.
The significant escalation is the latest boiling point in a long-standing conflict between Israel and Hamas.
Hamas is a Palestinian military and political group, gaining power in the Gaza Strip since winning legislative elections there in 2006.
Hamas' stated aim is to establish a Palestinian state, while refusing to recognise Israel's right to exist.
Hamas, in its entirety, is designated as a terrorist organisation by countries including Australia, Canada, the UK and the US.
Some countries list only its military wing as a terrorist group.
"We are fighting for our home. We are fighting for our future," Defence Minister Yoav Gallant said, meeting US Defence Secretary Lloyd Austin, who came to Israel a day after a visit by Secretary of State Antony Blinken.
"The path will be long but ultimately I promise you, we will win."
Austin said military aid was flowing into Israel but that this was the time for resolve and not revenge.
In the West Bank, demonstrators supporting Gaza fought gun battles with Israeli security forces.
Palestinian officials said 11 people were shot dead.
There have also been fears of hostilities spreading to new fronts, including Israel's northern border with Lebanon, where clashes this week have already been the deadliest since 2006.
