The United Nations has tightened security and ordered evacuation drills for its staff in Sudan as fears grow that an indictment of senior government officials over Darfur at the International Criminal Court could trigger violent reprisals.

Luis Moreno-Ocampo, the prosecutor of the ICC, will on Monday submit evidence on crimes committed in Darfur and seek the arrest of one or more senior members of the Khartoum regime, possibly including President Omar al-Bashir.