The UN Office for Project Services (UNOPS) said that one of its workers was killed Wednesday when an explosive was "dropped or fired" on its building in the central Gaza Strip.
The incident came after Israel launched its most intense bombardment of Gaza since a January 19 ceasefire, killing more than 400 people, according to the Health Ministry in the Hamas-run territory.
In a statement titled "UNOPS colleague killed at UNOPS premises in Gaza", the agency said: "An explosive ordnance was dropped or fired at the infrastructure and detonated inside the building" in Deir el-Balah city.
It added that "there is no confirmation at this stage as to the nature of the incident", which also injured five people, or of the type of ordnance.
Bulgaria's foreign ministry said on Wednesday that one its citizens working for the United Nations was killed in Gaza, without specifying where in the territory.
UNOPS chief Jorge Moreira da Silva said in a statement that he was "shocked and devastated" by the staff member's death.
"This was not an accident," he said, adding that "attacks against humanitarian premises are a breach of international law" and that UN personnel and premises "must be protected by all sides".
He called for the resumption of the ceasefire in the Israel-Hamas war, for "unimpeded humanitarian assistance" into Gaza, and for hostages held by Palestinian militants to be freed.
The Gaza health ministry earlier said a foreign UN worker was killed and five others seriously injured in an Israeli strike on their headquarters in central Gaza. The Israeli military denied hitting a UN building.
AFPTV footage from Deir el-Balah showed UN vehicles and an ambulance transporting three men to the Al-Aqsa Martyrs Hospital.
The images 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.
Two of the injured were wearing bulletproof vests, with one wearing a T-shirt of UNMAS, the United Nations Mine Action Service, underneath.
An Israeli military spokesperson told AFP that "there was no IDF operational activity" in the area of the UN compound and that "the IDF didn't strike" it.
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