Palestinians have been fleeing Gaza City's eastern Shejaiya district amid intense Israeli bombardment and a reported incursion by ground forces.
One Gaza City resident said it had "sounded as if the war is restarting", while Hamas-run authorities said air strikes had killed at least seven people.
Palestinian armed groups said they had targeted a tank and a bulldozer east of Shejaiya, where there were fierce battles during an Israeli operation at the end of last year.
The Israeli military has not commented on the reports, but it did order residents to evacuate and head southwards.
It comes days after Israel’s prime minister said that "the intense phase" of the fighting against Hamas was "about to end".
The Israeli military launched a campaign to destroy Hamas in response to an unprecedented attack on southern Israel on 7 October, during which about 1,200 people were killed and 251 others were taken hostage.
More than 37,760 people have been killed in Gaza since then, according to the territory's Hamas-run health ministry.
Shejaiya residents and Hamas-affiliated Palestinian media reported a series of Israeli air strikes, artillery shelling and helicopter fire on Thursday morning.
Later, they said Israeli ground forces had advanced into eastern areas of the district, which is not far from the Gaza-Israel perimeter fence.
A video posted on social media by activist Hema al-Khalili showed people running for cover after what he said was an air strike on a multi-storey building.
Other footage appeared to show hundreds of civilians fleeing the area on foot, many of them carrying their belongings, and several injured children being treated at a field hospital.
One person told the AFP news agency that the situation in Shejaiya was "very difficult and frightening".
"Residents are running through the streets in terror... a number of wounded and martyrs lie in the streets," they added.
Gaza's Hamas-run Civil Defence force reported strikes on several houses and said its rescue teams had recovered three bodies from the home of one family.
Later, it said the death toll in Shejaiya had risen to seven and that more casualties were feared to be under the rubble of destroyed buildings.
The Israel Defense Forces (IDF) did not put out a statement about Shejaiya.
However, it has mounted similar operations in northern Gaza in recent months in response to what it says is intelligence that Hamas fighters have regrouped there.
The IDF’s Arabic spokesman posted a message on social media telling residents to leave Shejaiya after the first strikes had been reported.
"For your safety, you must evacuate immediately south on Salah al-Din Street to the humanitarian zone," Lt Col Avichay Adraee said, referring to the north-south road used in the early weeks of the war by the hundreds of thousands of Palestinians who fled Gaza City when it was the initial focus of Israel's offensive against Hamas.
An attached map featured an arrow pointing south, but it was not clear whether Shejaiya residents were being instructed to head towards the "al-Mawasi humanitarian area" running along the coast in southern and central Gaza. That would entail crossing the east-west Israeli military zone south of Gaza City that effectively cuts the territory in half.
The al-Mawasi humanitarian area was expanded seven weeks ago, when Israeli ground forces began what they called "precise" operations in the southernmost city of Rafah, which they believe is Hamas’s last stronghold.
More than a million displaced people have fled Rafah since then, according to the UN, while the nearby Rafah border crossing with Egypt has been closed since Israeli troops seized control of the Palestinian side.
The UN says the crossing's closure has contributed to a significant reduction in the amount of aid reaching southern Gaza.
It has also prevented the medical evacuations of critically injured and sick Palestinians who require specialist treatment abroad.
On Thursday, 21 children with cancer were reportedly allowed to leave Gaza through the Kerem Shalom goods crossing with Israel for the first time, an Egyptian medical source told AFP.
There was no immediate confirmation from Israeli authorities, nor was it clear where they would receive treatment.
