Barack Obama at African Union for historic address

Barack Obama arrived at African Union headquarters Tuesday, where he will become the first US president to address the 54-member continental bloc, at the end of a tour focused on corruption, rights and security.

AU Commission chief Nkosazana Dlamini-Zuma hailed the "historic visit" as a "concrete step to broaden and deepen the relationship between the AU and the US".

Obama's address to the AU in the Ethiopia capital Addis Ababa is at its gleaming Chinese-built headquarters -- a symbol of Beijing's growing influence in the region.

He is widely expected to stress the importance of Africa's continued economic growth amid the challenges the continent still faces of ensuring equal rights, boosting democracy and ending corruption.

Obama is also expected to address Washington's strategic and security concerns, including Islamic extremism and the democratic deficit in many African nations.

The speech will close Obama's two-nation tour. After visiting Kenya, the country of his father's birth, Obama landed on Sunday in Ethiopia, where he praised the country as a key partner in the war against Al-Qaeda-affiliated Shebab in Somalia.

"Part of the reasons we've seen this shrinkage of Shebab in East Africa is that we've had our regional teams," Obama said Monday, referring to AU and Somali government troops.

"We don't need to send our own Marines in to do the fighting: the Ethiopians are tough fighters," Obama said, adding: "We've got more work to do. We have to now keep the pressure on."

After talks on Monday with Ethiopian Prime Minister Hailemariam Desalegn, whose ruling party won 100 percent of seats in parliament two months ago, Obama gave the blunt message that Ethiopia -- while credited with strong economic growth -- needed to improve basic rights.

Activists have complained that Obama's visit to Addis Ababa could add credibility to a government they accuse of suppressing democratic rights -- including the jailing of journalists and critics -- with anti-terror legislation.

Obama addressed those concerns, saying "there is still more work to do".

"There are certain principles we think have to be upheld," Obama added.

"Nobody questions our need to engage with large countries where we may have differences on these issues. We don't advance or improve these issues by staying away," he said.

It is a message he also pushed in Kenya, promoting the country's economic potential and vowing steadfast support for Nairobi's fight against the Shebab, but also telling Kenya to get tough on corruption and put an end to tribalism and gender discrimination.

- Catching up with China -

Ethiopia has come far from the global headlines generated by the 1984 famine, experiencing near-double-digit economic growth and huge infrastructure investment that have made it one of Africa's top-performing economies and a magnet for foreign investment.

Obama on Tuesday toured a US-supported food factory, where he spoke of the importance of supporting farmers to increase output.

"The goal is to drastically increase the productivity of small farmers all throughout Africa," Obama said. "With just a few smart interventions, a little bit of help, they can make huge improvements in their overall yield."

Obama has already held talks with regional leaders on the 19-month-old civil war in South Sudan, attempting to build African support for decisive action against the country's leaders if they reject an ultimatum to end the carnage by August 17, a new deadline set by regional mediators.

"On South Sudan, there was widespread unanimity about the urgency and severity of the situation on the ground," a US official said, after a meeting lasting nearly two hours.

South Sudan's rivals -- President Salva Kiir and rebel leader Riek Machar who were not invited to the meeting -- effectively face an ultimatum, a "final best offer", according to one senior administration official.

Signalling a deeper commitment to ending violence that has killed tens of thousands of people and forced more than two million from their homes, Obama is understood to have increased pressure for tougher sanctions and a possible arms embargo.

Obama also told reporters it was now time for a "breakthrough" in peace efforts.

South Sudan, midwifed into existence by US cash and support in 2011, has faltered badly in its infancy, leading to accusations that the Obama administration has abandoned the fragile nation.