DEIR AL-BALAH, Gaza Strip >> Israel said Wednesday its troops retook part of a corridor that bisects Gaza, and its defense minister warned that attacks would intensify until Hamas frees dozens of hostages and gives up control of the territory.

The military said it had retaken part of the Netzarim Corridor that divides northern Gaza from the south, and from where it had previously withdrawn as part of a ceasefire that began in January. That truce was shattered Tuesday by Israeli airstrikes that killed more than 400 Palestinians, mostly women and children, according to the Gaza Health Ministry.

The advances by Israel on Wednesday — which included sending more troops to southern Gaza — threatened to drag the sides into all-out war again. The ceasefire had given war-weary Palestinians some respite, allowed a much-needed surge of humanitarian aid into Gaza — and led to the release of dozens of hostages who had been held for more than 15 months.

Within Israel, the resumption of airstrikes has raised concerns about the fate of roughly two dozen hostages held by Hamas who are believed to still be alive.

A Hamas spokesman, Abdel-Latif al-Qanou, said the advance of ground forces in Gaza was a clear sign that Israel had backed out of the truce and was reimposing a “blockade.” There have been no reports of rocket attacks by Hamas since Tuesday’s bombardment.

Also Wednesday, the United Nations said one of its employees was killed in Gaza and five others were wounded in an apparent strike on a guesthouse. It was not immediately clear who was behind the strike, the U.N. said.

Ground operations in Gaza

The military said that its “limited ground operation” in Gaza would create a “partial buffer between northern and southern Gaza.”

It wasn’t immediately clear whether the move would entirely block Palestinians from traveling north or south through the Netzarim Corridor.

Israel used the roughly 4-mile corridor as a military zone during the war. It ran from the Israeli border to the coast, just south of Gaza City, severing the territory’s largest metropolitan area, and the rest of the north, from the south.

Israel continued with airstrikes in Gaza on Wednesday, though at a lower intensity than Tuesday, saying it hit dozens of militants and militant sites, including the command center of a Hamas battalion. It denied Palestinian claims that it hit the U.N. guesthouse.

Fares Awad, an official in the Gaza Health Ministry, said an Israeli strike on a gathering of mourners in the northern town of Beit Lahiya killed 14 and wounded 30 others. The Israeli military had no immediate comment.

Israel says it will order new evacuations

Hundreds of thousands of Palestinians who had fled northern Gaza for the south were prevented from returning throughout the war, until Israel withdrew from Netzarim in January. Many of them have since returned.

But Israel Katz, the Israeli defense minister, said the military would soon order Palestinians to evacuate from combat zones.

Katz said Tuesday’s aerial bombardment “was just the first step” in Israel’s plan to ratchet up the pressure on Hamas to release the remaining hostages and give up control of Gaza. Until it does, Israel will attack “with an intensity that you have not known.”

UN worker killed

Jorge Moreira da Silva, head of the U.N. Office for Project Services, declined to say who carried out the strike in the central city of Deir al-Balah. He said the blast was not accidental or related to demining activity.

He did not provide the nationalities of those killed and wounded.

Moreira da Silva said strikes had hit near the compound on Monday and struck it directly on Tuesday and again on Wednesday. He said the agency had contacted the Israeli military after the first strike. “Israel knew this was a U.N. premise, that people were living, staying and working there,” he said.

Lt. Col. Nadav Shoshani, a military spokesperson, said the explosion was not caused by Israeli fire. “There were no forces around that building, no aerial attacks on that area,” he said.

After the strike Wednesday, the wounded were rushed to Al-Aqsa Martyrs’ Hospital in central city of Deir al-Balah. One man was carried inside on a blanket held up by medical workers. Another lay on a hospital bed, his knee bandaged. A blue protective vest emblazoned with “UN” rested on a nearby bed.

The Gaza Health Ministry said at least 436 people, including 183 children and 94 women, have been killed since Israel launched the strikes early Tuesday. It said another 678 people have been wounded.