The renewed ground operations come a day after more than 400 Palestinians were reportedly killed in air strikes, shattering a ceasefire that has largely held since January.
The Israeli military said its operations have extended Israel's control over the Netzarim Corridor, which bisects the Gaza Strip, and were a "focused" manoeuvre aimed at creating a partial buffer zone between the north and the south of the enclave.
The United Nations said an Israeli air strike had killed a foreign staffer and wounded five workers at the site of a UN headquarters in central Gaza City on Wednesday.
But Israel denied this, saying it had hit a Hamas site where it had detected preparations for firing into Israeli territory.
Jorge Moreira da Silva, executive director of the UN Office for Project Services, said: "Israel knew that this was a UN premises, that people were living, staying and working there, it is a compound. It is a very well-known place."
UN Secretary-General Antonio Guterres called for a full investigation and condemned all attacks on UN personnel.
In a statement he said the strike brought to at least 280 the number of UN colleagues killed in the Gaza Strip since October 7, 2023.
In Sofia, the foreign ministry said a Bulgarian working for the UN died on Wednesday in the Gaza Strip, citing preliminary information.
It was not immediately clear if the Bulgarian was the foreign UN staffer killed.
Israel, which has vowed to eradicate Hamas, said its latest onslaught was "just the beginning".
In the latest violence, local health workers said an Israeli air strike killed four people and wounded 10 others in a house in the northern Gaza Strip town of Beit Hanoun, where the army issued evacuation orders earlier on Wednesday.
In Beit Lahiya, a new Israeli air strike killed 14 people at a mourning tent, medics said.
Israel and Hamas accuse each other of breaching the truce, which had offered a respite for the Gaza Strip's 2.3 million residents after 17 months of war that has reduced the enclave to rubble and forced most of its population to relocate multiple times.
The Israeli campaign has killed more than 49,000 people in the Gaza Strip, Palestinian health authorities say, and caused a humanitarian crisis with shortages of food, fuel and water.
Israel has accused Hamas of using Palestinian civilians as human shields.
Hamas denies this and accuses Israel of indiscriminate bombings.
The war - the most devastating episode in decades of Israel-Palestinian conflict - was triggered by a Hamas-led attack on southern Israel on October 7, 2023, in which gunmen killed 1200 people and took about 250 hostages, according to Israeli tallies.
Prime Minister Benjamin Netanyahu's decision to resume bombardments has triggered protests in Israel as 59 hostages are still held in the strip, with 24 of them believed to be still alive.
A coalition of hostage families and protesters against Netanyahu's moves against the judiciary and other parts of the security establishment has regrouped and accuses the prime minister of using the war for political ends.
Palestinian medics said Israeli tank shelling on the main north-south Salahuddin Road killed one Palestinian and wounded others.
Hamas spokesman Abdel-Latif al-Qanoua told Reuters the closure of the Salahuddin Road was a "total coup" against the ceasefire agreement and a tightening of the blockade on the Gaza Strip.
He renewed his group's interest in concluding the three-phased ceasefire agreement and would welcome any proposal "as long as it is based on launching negotiation on the second phase and a complete end to the war in Gaza".
Earlier on Wednesday, the Israeli army dropped leaflets in the northern and southern Gaza Strip, once again ordering residents to leave their homes.
Defence Minister Israel Katz issued a video statement warning Gaza Strip residents that evacuation of combat zones would begin shortly.
He said air strikes were "only the first step" and if the hostages were not released, "Israel will act with force you have not yet seen".
Katz urged the enclave's population to "remove Hamas".
The renewed violence was condemned by governments including France and Germany as well as Qatar and Egypt, which had been acting as mediators in the ceasefire negotiations.
with DPA
