US President Barack Obama landed in Ethiopia on Sunday, beginning a two-day stay and becoming the first American leader to visit Africa's second most populous nation.

Air Force One touched down at Addis Ababa's international airport after a short flight north from the Kenyan capital Nairobi, and the president was greeted on the tarmac by Ethiopian Prime Minister Hailemariam Desalegn.

The visit will include talks with the Ethiopian government, a key strategic ally but criticised for its record on democracy and human rights. Obama will also become the first US president to address the African Union, the 54-member continental bloc, at its gleaming, Chinese-built headquarters.

He will also hold talks with regional leaders on the civil war in South Sudan.

AU Commission chief Nkosazana Dlamini-Zuma hailed what she said will be an "historic visit" and a "concrete step to broaden and deepen the relationship between the AU and the US."

While Kenya launched one of the biggest security operations ever seen in the capital Nairobi to host Obama from Friday evening to Sunday, the habitual reach of Ethiopia's powerful security forces meant there was little obvious extra fanfare ahead of his arrival.

Ethiopia, like Kenya, has been on the frontline of the fight against the Somali-led, Al-Qaeda affiliated Shebab. Both nations have troops in Somalia as part of an AU and US-backed force, and are key security partners to Washington.

But the visit also comes two months after elections that saw the prime minister's ruling coalition take every one of the 546 seats in parliament.

The opposition, which lost its only seat, alleged the government had used authoritarian tactics to guarantee victory.

The US State Department notes Ethiopia's "restrictions on freedom of expression," as well as "politically motivated trials" and the "harassment and intimidation of opposition members and journalists."

Ahead of the visit, the White House stressed it frequently addresses issues of democracy and political rights with countries in the region. Having spoken frankly in Kenya on human rights and corruption, Obama can also be expected to address Ethiopia's -- and Africa's -- democracy deficit.

- South Sudan peace push -

Through the tinted windows of his bomb-proof presidential limousine, nicknamed "The Beast," Obama will see Addis Ababa's construction boom of tower blocks, as well as sub-Saharan Africa's first modern tramway.

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

The Horn of Africa nation also remains a favourite of international donors -- despite concerns over human rights -- as a bastion of stability in an otherwise troubled region.

On Monday Obama will try to build African support for tough action against South Sudan's warring leaders if they reject an August peace ultimatum.

Obama will meet with leaders from Ethiopia, Kenya and Uganda as well as Sudanese Foreign Minister Ibrahim Ghandour in Addis Ababa to try and build a collective front to end the 19-month-old civil war in the world's youngest nation.

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 expected to make the case for tougher sanctions and a possible arms embargo.

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

"The parties have shown themselves to be utterly indifferent to their country and their people, and that is a hard thing to rectify," the official said.

Rights groups, however, are not happy about Obama visiting Ethiopia, saying the trip could lend credibility to a government they accuse of suppressing democratic rights. They say that includes the jailing of journalists and critics, often using anti-terrorism legislation said to stifle peaceful dissent.

"We don't want this visit to be used to sanitise an administration who has been known to violate human rights," said Amnesty International's Abdullahi Halakhe, adding he feared Ethiopia would "spin" the visit to its benefit.