UNITED NATIONS, Dec. 20 (Xinhua) -- UN Secretary-General Antonio Guterres is outraged by the killing of World Food Programme (WFP) staff members in an aerial bombardment in Sudan, and calls for a thorough investigation, his spokesperson said Friday.

Three WFP personnel were killed when their office in Yabus, Blue Nile State was hit by airstrikes on Thursday, the agency confirmed on social media, and said they are gathering more information.

In a statement, Guterres sent his deepest condolences to the families of the victims and their colleagues.

The UN chief condemned all attacks on UN and aid personnel and facilities, and called for a thorough investigation.

"Yesterday's incident underscores the devastating toll that Sudan's brutal conflict is having on millions of people in need and the humanitarians trying to reach them with life-saving assistance," he said.

Guterres called on the parties to adhere to their obligations to protect civilians, including aid personnel, as well as humanitarian premises and supplies.

The UN chief once again urged the need for an immediate ceasefire in Sudan after more than 20 months of conflict. "The United Nations will continue to back international mediation efforts and work with all relevant stakeholders to help bring an end to the war," he said.

"Any loss of life in humanitarian service is unconscionable. Humanitarians are not, and must never be, a target," WFP Executive Director Cindy McCain said Friday, demanding a thorough investigation and "for the perpetrators to be held accountable."

The year 2024 is the deadliest on record for aid workers in Sudan, according to the United Nations. ■