The conflict, which erupted in 2023, has left behind a human toll which is "simply staggering", reports the BBC's Barbara Plett Usher.
As the conflict enters its fourth year, journalist Mohamed Suleiman shudders at what has been lost.
Victims describe how they have been targeted while carrying out everyday activities.
The strike killed 64 people, including 13 children, two nurses and a doctor, according to the WHO head.
President Idris Mahamat Déby orders the military to be on high alert and a "total closure" of the border.
A strong supply of weapons and a history of long wars make Sudan's conflict particularly troubling.
The RSF's capture of the city in October was one of the most brutal chapters of Sudan's nearly three-year civil war.
Intensified attacks in the Kordofan region have led to mass casualties as peace remains elusive.
A Chadian government spokesperson says the decision is aimed at protecting citizens.
