ADDIS ABABA, Ethiopia — Secretary of State Hillary Rodham Clinton bluntly warned African leaders on Monday that authoritarian governments ruled by aging despots were “no longer acceptable,” saying that those who refused democratic reforms would find themselves “on the wrong side of history.”

She also urged the African Union to end its lingering relations with Libya’s leader, Col. Muammar el-Qaddafi. American officials have been deeply frustrated by the organization’s efforts to mediate on behalf of Colonel Qaddafi, who for decades lavished support on African leaders — many of them autocratic — and led the group two years ago.

She also called for a peaceful resolution of the fighting that has flared in Sudan ahead of the planned declaration of independence by South Sudan on July 9.

The violence, in the disputed territory of Abyei and increasingly in other regions along what will be the new border, has threatened to unravel a peaceful separation that the Obama administration worked feverishly to ensure over the last year. Mrs. Clinton called the recent fight “deeply troubling.”