President Barack Obama, wrapping up the first leg of an African tour, said Washington had a "moral imperative" to help the world's poorest continent feed itself.

White House officials hope Mr Obama's three-nation tour of Africa – his first substantial visit to the continent – will compensate for what some view as years of neglect by America's first black president.

Before departing Senegal, Mr Obama met farmers and entrepreneurs to discuss new technologies that will raise agricultural output.The President said his administration was making food security a top priority of its development agenda.

He added: "This is a moral imperative. I believe Africa is rising and wants to partner with us: not be dependent but be self sufficient. Far too many Africans endure the daily injustice of poverty and chronic hunger."

Mr Obama played down expectations of a meeting with ailing Nelson Mandela.

He said: "The last thing I want to do is to be in any way obtrusive at a time when the family is concerned with Nelson Mandela's condition."

Mr Obama is scheduled to visit Robben Island, where Mr Mandela spent years in prison under South Africa's former white minority regime, later during his trip and the last stop will be Tanzania.

The President's only previous visit to the continent was a one-day stopover in Ghana.

Food security, along with anti-corruption measures and trade opportunities for US companies, are topics the White House wants to highlight on Mr Obama's eight-day tour.

While acknowledging Mr Obama has not spent as much time in Africa as people hoped, the administration is eager to highlight what it has done, in part to end unflattering comparisons to accomplishments of predecessors George W. Bush and Bill Clinton.

Food security and public aid are two of the issues the Obama team believes are success stories. USAID head Raj Shah told reporters Africa had seen a steady increase in resources under Mr Obama's administration despite a tough budget environment.