US President Barack Obama has strongly criticized African leaders who refuse to relinquish power and cause instability during the first address by an American president to the African Union.  

"Africa's democratic progress is also at risk when leaders refuse to step aside when their terms end," Obama said in a speech on Tuesday at the AU's headquarters in Ethiopia’s capital, Addis Ababa.

"No one should be president for life," Obama said, adding that he was looking forward to handing power over to his successor.

"I have to be honest with you: I just don't understand this. Under our constitution, I cannot run again. There's still so much I want to get done to keep America moving forward. But the law is the law and no one is above it, not even presidents," he said.

"And, frankly, I'm looking forward to life after being president. It will mean more time with my family and more visits to Africa."

Obama singled out the actions of Burundi's President Pierre Nkurunziza as an example of the dangers of trying to stay in power. Nkurunziza’s re-election to a third term provoked weeks of unrest in the small central African nation.

"When a leader tries to change the rules in the middle of the game just to stay in office, it risks instability and strife, as we've seen in Burundi," the US leader said, adding that hanging on to power was "often just a first step down a perilous path."

"If a leader thinks they're the only person who can hold their nation together, then that leader has failed to truly build their country," Obama said, hailing former South African president Nelson Mandela as an example for the continent.

Obama flew to Ethiopia after a two-day visit to his father’s homeland in Kenya where he had discussed trade and security. He will depart Africa and travel back to Washington on Tuesday.