An injured foreign UN staff member is transported in an ambulance to the Al-Aqsa Martyrs Hospital in Deir el-Balah in the central Gaza Strip on March 19, 2025. AFP
Israel has denied responsibility for the attack, which also injured five others.
Guterres "was deeply saddened and shocked to learn of the death of a United Nations Office for Project Services staff member, when two UN guesthouses in Deir el-Balah were hit in strikes," Farhan Haq told the press at UN headquarters in New York.
He added that the attack brings the number of UN employees killed in Gaza since October 7, 2023 to at least 280.
The deaths took place amid a renewed Israeli offensive in Gaza which the Gaza health ministry says has killed more than 400 people in two days, by far the highest toll since a fragile ceasefire came into force on January 19.
A UN source had earlier said a second employee was killed. Haq said that was not the case, but that one of the five injured employees was in very critical condition. He did not provide the nationality of the dead employee.
"The locations of all UN premises are known to the parties to the conflict, who are bound by international law to protect them and maintain their absolute inviolability," Haq said.
"The secretary-general strongly condemns all attacks on UN personnel and calls for a full investigation," he added.
Haq said it was too early to "determine responsibility," but that the explosion was not the result of a mine or unexploded ordnance, but a projectile that was "either dropped or fired at the infrastructure and detonated inside the building."
"We do not know whether it was fired from land, from the sea, from the air," he said.
The Gaza health services had said earlier Wednesday that the UN worker was killed by an Israeli strike.
But Israel denied it had carried out any attacks on Deir el-Balah, in the center of the Palestinian territory.
"Contrary to reports, the IDF (army) did not strike a UN compound in Deir el-Balah," said a statement, while an army spokesperson told AFP that "there was no IDF operational activity there and that the IDF didn't strike the UN compound."
Jorge Moreira da Silva, head of the UN Office for Project Services (UNOPS), said "this was not an accident."
AFPTV footage from the Al-Aqsa hospital showed two men who appeared to have leg injuries and a third with bandages on both arms and abdomen and traces of blood on his chest being brought in for treatment.
Medics and UN staff stand outside the Al-Aqsa Martyrs Hospital, after transporting casualties from UN staff members, including foreigners, to the medical facility in Deir el-Balah in the central Gaza Strip. AFP
Two of the injured were wearing bulletproof vests, with one wearing a T-shirt of UNMAS, the United Nations Mine Action Service, underneath.
Hamas accused Israel of "aiming to terrorise" civilians and aid workers in Gaza.
"This crime is not only a blatant violation of international humanitarian law but also part of (Israel's) systematic policy of targeting civilians and aid workers, aiming to terrorise them and prevent them from fulfilling their humanitarian duty toward our people," a Hamas statement said.
On Tuesday, Israel resumed its genocidal war on Gaza, killing at least 450 Palestinians, including more than 170 children and 80 women, according to the United Nations Office for the Coordination of Humanitarian Affairs (OCHA) and the Palestinian Civil Defence.
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