The US president has arrived in Ghana for a visit that is expected to see him discuss democracy and poverty.

Archbishop Tutu says he should use his influence as the first African American US head of state to warn governments they must be more accountable to their people.

"He is one of the few Western leaders who can tell them to pull their socks up very, very firmly," the Archbishop told Sky News.

"I would be disappointed if he didn't refer to the fact that many African countries are languishing because of poor governance."

Mr Obama, whose father came from Kenya, has conspicuously avoided choosing that nation - still in a state of political upheaval after last year's disputed elections - for his maiden African tour.

By contrast Ghana, the first sub-Saharan African nation to achieve independence, can claim to be a stable democracy and will be held up as an example of an African success story by Mr Obama.

The West African nation was once a hub of the transatlantic slave trade.

The President and the First Lady, Michelle Obama - a descendant of a slave - are expected to visit one of the old forts which represented the 'point of no return' for hundreds of thousands of Africans who were forcibly shipped to the US.

President George Bush made few breakthroughs in dealing with conflict and poor leadership on the continent.

But he is credited with huge successes in the delivery and targeting of American aid.

His PEPFAR initiative - the President's Emergency Plan for Aids Relief - pumped more American government money into Africa than ever before.

The programme has made life-saving anti-retroviral drugs (ARVs) widely available in 13 countries across the continent, including South Africa.

Doctors there say Aids treatment has been revolutionised as a result.

"HIV used to be a death sentence, but because we now have ARVs, people are recovering," said Dr Patrick Mashele, a physician at the Soweto Hospice.

When PEPFAR was launched, only 50,000 people were taking ARVs in sub-Saharan Africa. Today that figure is 2.1 million - largely as a result of US funding.

President Obama has already pledged funding for PEPFAR for another five years.

He can also point to the G8's commitment to provide US$15bn to tackle food insecurity in the world's poorest nations as evidence that poverty in Africa remains high on America's agenda.