The Israeli military has begun a ground offensive against Palestinian militants in the Gaza Strip, extending its 10-day-old Operation Protective Edge.
It said it was in response to continued militant rocket fire and to strike a "significant blow to Hamas", which controls Gaza.
Hamas said Israel would pay a high price for the ground offensive.
There had been a five-hour humanitarian truce on Thursday, but exchanges of fire resumed when it ended.
Some 230 Palestinians and one Israeli have died during the Operation Protective Edge period.
At least five Palestinians were killed after the ground offensive began, Palestinian authorities said.
They included a five-month-old in the town of Rafah, medical officials told AFP news agency.
Israel withdrew ground troops from the Gaza strip in September 2005, and last mounted a major ground operation in 2009.
In a statement, the Israeli military said: "Following 10 days of Hamas attacks by land, air and sea, and after repeated rejections of offers to de-escalate the situation, the IDF (Israel Defense Forces) has initiated a ground operation within the Gaza Strip."
It said the goal was to "establish a reality in which Israeli residents can live in safety and security without continuous indiscriminate terror, while striking a significant blow to Hamas' terror infrastructure".
Military spokesman Gen Moti Almoz said: "I ask the residents of Gaza to evacuate the areas in which the army is operating. This operation will be extended as much as necessary."
Israel approved the drafting of 18,000 more reservists on Thursday evening, bringing the total of extra troops called up since 8 July to 65,000.
Hamas chief Khaled Meshaal said that the Israeli ground operation was "destined to failure".
"What the occupier Israel failed to achieve through its air and sea raids, it will not be able to achieve with a ground offensive", he said.
An intensive bombardment of Gaza by Israeli planes, artillery and ships has been taking place.
Media in Gaza have reported seeing Israeli commandos on the beach there.
The BBC's Yolande Knell in Gaza says journalists were warned to take shelter, shortly before the ground offensive announcement was made.
Other Gaza witnesses say 10 tanks have crossed the border into north-west Gaza.
More rockets were fired from Gaza into southern Israel after the operation began, Israeli authorities said.
Israeli media said one key aim of the ground offensive was to destroy tunnels that could be used to infiltrate militants into Israel.
An attempt by 13 militants to tunnel into Israel to attack a kibbutz on Wednesday had been thwarted by the Israeli military.
The BBC's Kevin Connolly in Jerusalem says it is possible that with the ground invasion Israel is seeking to improve its military position in advance of any deal, but that more probably it has concluded that a ceasefire at the moment in unlikely.
Israel says it has carried out more than 1,960 attacks on Gaza since 8 July, while militants have fired some 1,380 rockets at Israel.
The UN says at least 1,370 homes have been destroyed in Gaza and more than 18,000 people displaced in recent hostilities.
It says most of those killed in Gaza have been civilians.
The ground offensive follows attempts in Cairo to negotiate a new ceasefire.
There had been some reports that a new truce would start at 06:00 local time (03:00 GMT) on Friday.
However, both Israel and Hamas later dismissed the reports.
Thursday's temporary ceasefire had been requested by the UN and other international organisations to provide emergency relief and distribute water, food and hygiene kits to the people of Gaza.
The truce lasted from 10:00 and 15:00 local time, although both sides reported violations.
But almost immediately after it ended, Israel reported renewed militant rocket fire and there were fresh air strikes on targets in Gaza.
The health ministry in Gaza said one Israeli air strike had hit a home in the Sabra area of Gaza City on Thursday, killing three children - aged between seven and 10 - from the same family.
Credit: The BBC