Frederic Mar, from Aide Médicale Internationale, said the bus the aid workers were on was stopped by the armed group, who opened fire when the passengers panicked, killing two members of staff and injuring four civilians.

Tension has increased in Darfur as the International Criminal Court (ICC) considers whether to issue an arrest warrant for Omar al-Bashir, the Sudanese president, for alleged war crimes.

ICC judges are expected to rule on a decision within weeks.