JERUSALEM/UNITED NATIONS - Israel on Wednesday expressed "sorrow" over the death of a Bulgarian United Nations staff member in strikes on UN guesthouses in Gaza's Deir al-Balah but said an initial investigation found no Israeli involvement in the incident.

The United Nations said the staffer, employed by the UN Office for Project Services (UNOPS), was killed when two UN facilities were hit. Five other personnel sustained serious injuries.

Israeli Foreign Ministry spokesperson Oren Marmorstein said a preliminary inquiry "found no connection" between Israeli military operations and the strike.

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"The circumstances of the incident are being investigated," he said, adding that Israel facilitated the evacuation of the victim's body and the wounded from the site.

The injured would receive treatment in Israeli hospitals, he said in a post on social media platform X.

UN Secretary-General Antonio Guterres condemned the attack as a violation of international law. "All UN premises' locations are known to parties to the conflict, who are obligated to protect them," a UN spokesperson said in a statement.

Guterres extended condolences to the victim's family, noting that the death brought the number of UN staff killed in Gaza since Oct 7, 2023 to at least 280.

"These premises were well known by the Israel Defense Forces (IDF) and they were 'deconflicted'," UNOPS chief Jorge Moreira da Silva said, adding that it was clear UN personnel were inside at the time.

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"This was not an accident, this was an incident," he told reporters. "What's happening in Gaza is unconscionable. I am shocked and devastated by this tragic news."

UNOPS said in a statement that the explosion, which took place at around 11:30 am Wednesday local time following Tuesday's strikes, caused damage to the premises.

"There is no confirmation at this stage as to the nature of the incident, however the location is isolated," it said.

"What we do know is that an explosive ordnance was dropped or fired at the infrastructure and detonated inside the building," UNOPS said. "We don't know at this stage what type it was (airdrop weapons, artillery, or rocket). It should be stressed that it was not through any action removing unexploded ordnance at the location."

Three of the injured UNOPS personnel worked in support of the UN Mine Action Service Occupied Palestinian Territory program. Two others worked in support of the UN 2720 Mechanism for Gaza.

The head of UNOPS stressed that attacks against humanitarian premises are a breach of international law, and called for the resumption of the ceasefire.

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Earlier on Wednesday, the Israeli military denied targeting the compounds. "Contrary to reports, the IDF did not target a UN facility in Deir al-Balah," it said in a statement.

The incident occurred as Israel renewed military operations in Gaza, which officials say are aimed at Hamas militants. Gaza health authorities reported more than 400 Palestinian deaths since Tuesday, including at least 170 children and 80 women, after Israel resumed strikes following a weeks-long ceasefire that began on Jan 19.

Israel's military said the operations were intended to "eliminate Hamas threats" and would continue "until strategic objectives are achieved".  

Acting UN special adviser on the prevention of genocide, Virginia Gamba, and the special adviser on the responsibility to protect, Mo Bleeker, said they are gravely concerned by reports of extensive Israeli airstrikes across the Gaza Strip on Tuesday.

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They said the developments signal a troubling and dramatic escalation of violence with irreversible consequences. It is essential that the mutual imperatives of the peace process, integrating aspects of prevention and protection are prioritized urgently.

They urged all parties to prioritize the protection of civilians and take immediate steps to de-escalate tensions, prevent further loss of life and engage in a solid political solution.