ADDIS ABABA (Reuters) - The Red Cross has halted operations across a third of South Sudan after gunmen shot dead a staff member, in what the U.N. said on Wednesday was the biggest such suspension during the country’s four-year civil war.

Kennedy Laki Emmanuel, a driver for the Red Cross, died on Sept. 8 when gunmen fired on a 10-vehicle convoy delivering aid in South Sudan’s restive Western Equatoria state.

In response, the International Committee of the Red Cross shut down activities across Equatoria, a region roughly the size of Britain that borders Congo and Uganda and has seen some of the heaviest fighting over the last year.

The suspension affected more than 22,000 people about to get aid deliveries from the Red Cross. That included more than 5,000 farmers due to receive seeds in an area teetering on the edge of famine.

“The ICRC will not resume anything until we have a clear picture of exactly what happened and until we receive the necessary security guarantees,” spokeswoman Mari Mortvedt told Reuters. “The security of the ICRC staff is top priority.”

The U.N.’s Office for Coordination of Humanitarian Affairs told Reuters that no aid group had shut down its operations over such a large area since South Sudan’s civil war began in 2013.

The conflict began after President Salva Kiir, an ethnic Dinka, fired his deputy Riek Machar, an ethnic Nuer. The U.N. says ethnic cleansing has taken place and warned of genocide, amid reports of murder, rape, and torture of civilians.

More than two million South Sudanese have now fled the country, creating Africa’s biggest refugee crisis since the 1994 Rwandan genocide, and more than half of those remaining need food aid. The country’s original population was 12 million.

At least 85 aid workers have been killed, according to the U.N., including 18 this year, making it the deadliest country for aid workers in the world.

That number did not include a staff member for aid group World Vision, killed in the Western Equatoria town of Yambio on September 3. It was not clear whether that death was related to his work as an aid worker, so it was not included in the total, the group said.

(This story corrects to make clear World Vision aid worker not included in total of 85)