Israel says it has extended its ground operations in Gaza, after launching a wave of air strikes that the Hamas-run health ministry says killed 970 people in two days.
The Israel Defense Forces (IDF) said troops had moved in up to the Netzarim Corridor, which divides the north and south of the Strip, BBC, Press TV reported.
The fresh assault on Gaza marks the end of the fragile ceasefire deal that had been in place since January.
Earlier, the UN said two people - including one of its staff - had been killed after an explosion at its compound in Deir al-Balah.
A foreign ministry spokesman said it would launch an investigation, but denied Israel was to blame.
The IDF said it had begun "targeted ground activities" to create what it called a "partial buffer between the north and south" in Gaza.
It has issued numerous orders to Gazans to leave a huge area along the three sides of the territory's land borders, suggesting that a larger ground operation could shortly take place.
The orders sent panic among Palestinian families, many of whom had been displaced repeatedly by the war and had returned home during the ceasefire.
Families have been seen carrying what they could and leaving by foot, by cart or vehicle, to seek safety away from the areas marked by the Israeli military.
In a video message on Wednesday, Israeli Defence Minister Israel Katz issued a "last warning" to the Palestinian territory calling for the return of the remaining hostages being held there.
Israel says Hamas is still holding 59 hostages, 24 of whom are believed to be alive.
Katz also repeated the call for the end of Hamas, saying if neither demands are met the alternative would be "total destruction and devastation".
The movement of troops comes after the UN said a member of staff had been killed when its compound in Deir al-Balah was damaged on Wednesday.
The UN Office for Project Services (UNOPS) said an "explosive ordnance was dropped or fired" at the building, which was in an "isolated" location. It added there was no confirmation on the nature of the incident or the type of artillery used.
Meanwhile, UN Secretary General António Guterres has called for a full investigation.
bd-pratidin/GR
