US President Barack Obama opens business on a long-awaited African tour on Thursday by paying homage in Senegal to the innocents forced into the slave trade.

The week-long, three-nation visit is meant to finally honor unfulfilled hopes for the United States' first black president in Africa, but is being overshadowed as anti-apartheid icon Nelson Mandela nears death.

Obama is due to travel on to South Africa on Friday, and then to visit Tanzania, but Mandela's death before then would likely cause radical changes to his itinerary.

The US president stepped off Air Force One into the African night on Wednesday with his wife, Michelle, and daughters Sasha and Malia, and was scheduled to meet Senegalese President Macky Sall on Thursday, before holding a news conference.

His motorcade sped through streets cleared by police, as a local radio station played a musical arrangement of one of his 2008 "Yes We Can" campaign speeches, evoking better days during Obama's increasingly grim second term.

In a highly symbolic moment, Obama was scheduled on Thursday to take a ferry to the House of Slaves on Goree Island, which memorializes the hundreds of thousands of Africans swept into the slave trade.

It's the first of two island visits Obama has planned for the trip to highlight racial atrocities of the past. The second is scheduled for Sunday at South Africa's Robben Island, where anti-apartheid leader Nelson Mandela was imprisoned for 18 years.

"A visit like this by an American President, any American President, is powerful," White House spokesman Jay Carney told reporters on Air Force One, when asked about Obama's visit to Goree off the Senegalese coast.

"I think that will be the case when President Obama visits and I'm sure particularly so, given that he is African-American."

Obama's arrival in Africa came at a deeply poignant time as the world prepared to say a farewell to former South African president Mandela.

White House officials say they are monitoring Mandela's condition and praying for him, but his plight looks increasingly likely to complicate plans for Obama, who is due to spend the weekend in South Africa.

Obama and Mandela met in 2005, when the former South African president was in Washington, and Obama was a newly elected senator, and the two have spoken several times since by telephone.

But the long awaited prospect of a public appearance between the first black presidents of South Africa and the US is now impossible.

Obama, whose late father was from Kenya, claims a spiritual connection to Africa, but a crush of international crises in his first term thwarted his hopes to travel extensively in the continent. He did manage a short trip to Ghana in 2009.

His tour is designed to highlight Africa's emerging economic potential and growing middle class, as well as youth and health programs, and to emphasize US engagement in a region benefiting from a wave of Chinese investment.

"We are not too late," said Carney, pointing out that although Obama had been kept away, Vice-President Joe Biden visited Africa in the first term, and there were also wide-ranging diplomatic efforts by the administration on the continent.

But there has been disappointment in Africa, after Obama's 2008 election caused euphoria and an expectation that he would put Africa policy at the top of his agenda.

The current US president also travels in the shadow of his predecessors, Bill Clinton and George W. Bush, who are remembered fondly for their economic development and HIV/AIDS programs.

AFP - AP