Obama, Cuban President Raul Castro Shake Hands at Nelson Mandela Memorial

Dec. 10, 2013— -- President Obama shook hands with Cuban President Raul Castro today at a memorial service for the late Nelson Mandela in South Africa.

Obama stopped to shake Castro's hand as he was walking to the podium to pay tribute to Mandela. The handshake between the presidents of the long-estranged Cold War rivals came at a ceremony honoring the life and legacy of the former South African leader at a soccer stadium in Johannesburg.

VIDEO: Obama, Castro Shake Hands at Mandela Memorial

It's been more than 50 years since the U.S. cut diplomatic ties with Cuba following the Communist revolution led by Castro's brother, Fidel. While there have been glimmers of a thaw in relations in recent years, vast obstacles remain.

The Obama administration continues to call for the release of an American man who has been imprisoned in Cuba for four years. Alan Gross, a 64-year-old husband and father, was sentenced to 15 years in prison for facilitating uncensored Internet contact between a Cuban religious community and the rest of the world.

LIVE UPDATES: World Leaders Remember Mandela

Making his way to the podium, Obama also shook hands with Brazilian President Dilma Rousseff, who earlier this year cancelled a state visit to Washington over concerns about alleged spying by the U.S.

Castro's speech will be followed by the keynote address scheduled to be given by South African President Jacob Zuma.