U.N. Secretary-General Ban Ki-moon called Sudan's agreement on a large force for Darfur "a milestone" on Wednesday, but U.N. envoys acknowledged challenges on command structures and finding enough troops. "It was a milestone development," Ban said. "Even though slow, we have been making steady progress ... and we are now moving toward the right direction." Sudan accepted the agreement in Addis Ababa, Ethiopia, on Tuesday, Reuters reported. Jean-Marie Guehenno, the undersecretary-general for peacekeeping, backed Sudan's contention that Khartoum would accept non-African troops in the combined U.N.-African Union force of up to 23,000 soldiers and police.