Civilians have now been ordered out of the majority of the Gaza Strip's largest city, where thousands of families had taken shelter from fighting in other parts of the war-stricken territory.
Backed by fighter jet and drone attacks, columns of tanks took up positions around the city, while an army spokesman warned residents of the Sabra, Rimal, Tal Al-Hawa and Al-Daraj districts to flee to so-called "humanitarian zones".
The civil defence agency in the Hamas-run territory said even before the additional warning that it had reports of "dozens" of dead and wounded from night-time fighting in different parts of the city.
The Israeli military has been fighting for nearly two weeks in the Shujaiya district of Gaza City where it issued an earlier evacuation order. It said it had "eliminated" more than 30 "terrorists" in the latest fighting there.
Loudspeakers on armoured vehicles toured the Al-Daraj and Al-Tuffah neighbourhoods telling people to leave.
Israel said the operation targets included the headquarters of the UN agency for Palestinian refugees, UNRWA, in Gaza City.
The military said it was acting "following intelligence indicating the presence of Hamas and Islamic Jihad terrorist infrastructure, operatives, weapons, and investigation and detention rooms in the area of Gaza City, including in the UNRWA headquarters."
Fleeing residents reported heavy fighting. Muhammad Bisan said he had been through "an indescribable night".
"Planes and artillery are bombing and drones are firing from all directions, and we do not know where to run, right or left," he told AFP.
Some said they had only just left other areas after being ordered out by the army.
"Where do we go," said Abdullah Khammash, who told how he left his latest refuge at three in the morning.
"We ran away ... and slept in the streets. People felt sorry for us and gave us blankets and here we are again living in the rubble."
Districts near the southern cities of Khan Yunis and Rafah are also under evacuation orders.
