WASHINGTON (AFP) - The White House said on Monday it was monitoring Nelson Mandela's condition, but could not yet say whether his failing health would affect President Barack Obama's trip to Africa this week.

Obama is due to travel to Senegal on Wednesday and spend the weekend in South Africa before journeying on to Tanzania, on a keenly awaited first big Africa tour of his presidency.

But there has been speculation that if Mandela, 94, passes away in the next few days, the entire trip would be either cancelled or substantially reworked.

"We are monitoring the situation and understand from the reports that former South African President Nelson Mandela is in critical condition," said White House spokesman Jay Carney.

"I wouldn't want to speculate about the impact of Mr Mandela's health on the president's trip.

"He continues to look forward to the trip and to his visit to South Africa and to continuing to build on our already very strong partnership with the South African government and people."

Carney reiterated that America's thoughts and prayers were with Mandela and that Obama has long seen him as a political hero.

"We're all wishing for his recovery." South African President Jacob Zuma, whom Obama is due to meet, said the trip, which includes a visit by the president to Mandela's former jail cell on Robben Island, will go on.

But in the event of Mandela's death, the focus would quickly turn to the US president's likely attendance at what is expected to be an elaborate funeral ceremony.

A meeting between the first black presidents of South Africa and the United States has long been anticipated, but given Mandela's current condition, now appears impossible.

The two men did meet briefly in 2005, when ex-president Mandela was in the US capital and Obama was a newly elected US senator.

The White House said last week it would defer to Mandela's family as to whether there would be any interaction between the president and the former South African leader, who is in hospital in Pretoria.