WASHINGTON - US President Barack Obama and his two predecessors, George W. Bush and Bill Clinton, will all travel to South Africa next week for memorial events in honor of Nelson Mandela, the White House said Friday.

Obama and his wife, Michelle, will be joined on Air Force One by Bush and his wife, Laura, on the trip to South Africa.

Clinton has also stated his intention to attend Mandela's memorial, but his exact travel plans are still being worked out, according to a spokesman.

The exact schedules of Obama, Bush and Clinton remain unclear, and it is not known whether Obama will make public remarks in South Africa.

Obama called the widow of former South African President Nelson Mandela on Friday to express his condolences and gratitude for the anti-apartheid icon's influence on him.

In his phone conversation with Graca Machel, Obama thanked her "for the profound influence that Mandela has had on him, and underscored the power of President Mandela's example for the people of South Africa and the entire world," the White House said in a readout of the talks.

Obama also "expressed gratitude and thanks for the joy that Graca Machel brought to Mandela's life, and the commitment to a peaceful, fair and loving world that she and President Mandela shared," the White House said.

Mandela, the first black president of South Africa from 1994 to 1999, died of prolonged lung infection on Thursday at his home in Johannesburg at the age of 95.

In a televised statement at the White House on Thursday, a saddened Obama mourned the death of Mandela, his "personal hero," saying, "he achieved more than could be expected of any man."

In a proclamation signed later that day, Obama ordered an immediate lowering of flags for Mandela at the White House, upon all public buildings, at all military posts and foreign missions until sunset of next Monday.

As the first black president in US history, Obama never met Mandela face-to-face in his presidency. Their only meeting occurred in 2005, when Obama was a US senator.

When Obama visited South Africa in June, the iconic leader was hospitalized with a recurring lung infection. Obama and his family visited the jail cell on Robben Island where Mandela was held for 18 of his 27 years in prison during South Africa's apartheid era.

South African President Jacob Zuma announced Friday a period of mourning for Mandela, with a state funeral and burial slated for December 15.