DEIR AL-BALAH, Gaza Strip — An international United Nations staffer was killed and five others were wounded in a strike Wednesday on a U.N. guesthouse in the Gaza Strip, as Israel pressed on with a new offensive in the territory that shattered a fragile ceasefire with Hamas.

Israeli Defense Minister Israel Katz threatened to step up the assault, warning Palestinians in Gaza that Israel would again order evacuations from combat zones soon. He said that if hostages held in the territory weren’t freed, “Israel will act with an intensity that you have not seen.”

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 but said the explosive ordnance was “dropped or fired” and the blast was not accidental or related to demining activity.

He did not provide the nationalities of those killed and wounded. The U.N. body, known as UNOPS, carries out infrastructure and development projects around the world.

Israel Denies Targeting UN Compound

The Israeli military, which has carried out a heavy wave of airstrikes throughout Gaza since early Tuesday, denied earlier reports that it had targeted the U.N. compound.

But Moreira da Silva said strikes had hit near the compound on Monday and struck it directly on Tuesday and again on Wednesday, when the staffer was killed. He said the agency had contacted the Israeli military after the first strike and confirmed that it was aware of the facility’s location.

“Israel knew this was a U.N. premise, that people were living, staying and working there,” 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.

There have been no reports of rocket fire or other Palestinian militant attacks since Israel unleashed the airstrikes overnight and into Tuesday, ending a ceasefire with Hamas that took hold in January. The Israeli bombardment continued into Wednesday, though at a lower intensity.

Gaza Health Ministry Said 436 People Dead, Including 283 Children

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.

The military says it only strikes militants and blames civilian deaths on Hamas because it operates in densely populated areas. Gaza’s Health Ministry records do not distinguish between civilians and combatants.

The military said in a statement that as part of the new offensive, it struck dozens of militants and militant sites on Wednesday, including the command center of a Hamas battalion.

The war in Gaza, which was paused in January by an internationally-mediated ceasefire agreement between Israel and Hamas, has been among the deadliest conflicts ever for humanitarian workers, according to the U.N.

The resumption of fighting launched by Israel early Tuesday risks plunging the region back into all-out war. It came weeks after the end of the first phase of the ceasefire, during which Israel and Hamas exchanged hostages for prisoners and were set to negotiate an extension to the truce that was meant to bring about an eventual end to the war.

But those negotiations never got off the ground. Hamas has demanded that Israel stick to the terms of the initial ceasefire deal, including a full withdrawal from Gaza and an end to the war. Israel, which has vowed to defeat Hamas, has put forward a new proposal that would extend the truce and free more hostages held by Hamas, without a commitment to end the war.