ADDIS ABABA, Ethiopia — Making his first official trip to sub-Saharan Africa, U.S. Secretary of State John Kerry on Saturday demanded that Nigeria respect human rights as it cracks down on Islamist extremists and pledged to work hard in the coming months to ease tensions between Sudan and South Sudan.

Kerry, attending the African Union's 50th anniversary, backed the Nigerian government's efforts to root out Boko Haram, an al-Qaida-linked radical sect. But he said there is no excuse for abuses by armed forces in Nigeria's long neglected north, where President Goodluck Jonathan has declared emergency rule.

"We defend the right completely of the government of Nigeria to defend itself and to fight back against terrorists," Kerry said. He added, however, that he has raised his concerns with Nigerian officials to insist on the military "adhering to the highest standards and not itself engaging in atrocities."

"One person's atrocities do not excuse another's," Kerry said. "Revenge is not the motive. It's good governance, it's ridding yourself of a terrorist organization so that you can establish a standard of law that people can respect."

Speaking to reporters alongside Ethiopian Prime Minister Hailemariam Desalegn, Kerry also blamed Sudan's government for much of the tension along its volatile border with South Sudan. He says residents in the contested areas of Blue Nile and South Kordofan don't want to be subjected to strict Islamist rules.

Both areas border the new nation of South Sudan, which gained independence in 2011 under an agreement that ended decades of civil war. Many residents are sympathetic to the South, and both areas have experienced regular violence in recent years.

"There are very significant border challenges, but they're bigger than that," Kerry said. "You have people who for a long time have felt that they want their secular governance and their identity respected."

"They don't want independence; they are not trying to break away from Sudan," he said. But he said the response from Sudan's government has been to "press on them through authoritarian means and violence an adherence to a standard that they simply don't want to accept with respect to Islamism."

"That's the fundamental clash," Kerry said.

He acknowledged, however, the North's concerns that the South is fueling rebels in the areas and said the U.S. would try to work with Ethiopia and other international partners to ease tensions. He said he'd soon appoint a new American envoy to both countries.

Kerry meets the foreign ministers of both Sudans later Saturday.

His meetings in Ethiopia's capital also include the U.N. and African Union chiefs and Egyptian President Mohammed Morsi.