President Obama's unprecedented handshake with Cuban President Raul Castro today was not a pre-planned encounter, a White House official told ABC News.

"Above all else, today is about honoring Nelson Mandela, and that was the President's singular focus at the memorial service," the official said. "We appreciate that people from all over the world are participating in this ceremony. As the president said, we urge leaders to honor Mandela's struggle for freedom by upholding the basic human rights of their people."

The leaders of the long-estranged Cold War foes had a brief encounter at a memorial service honoring the life and legacy of former South African leader Nelson Mandela in Johannesburg. Obama and Castro, who both delivered remarks at the ceremony, shook hands and spoke to one another on the podium.

Sen. Marco Rubio, a Florida Republican and fierce critic of the Castro regime in Cuba, told ABC News that he didn't have a problem with Obama shaking Castro's hand.

"I'm more focused on policy than some moment," Rubio said in a brief interview. "But if he was going to be there and shake his hand, he should have asked him about those basic freedoms that Mandela is associated with that are systematically denied in Cuba."

Rubio, whose father fled Cuba, believes the Obama administration should be taking a harder line with Castro.

"My biggest problem," he said, "is not the handshake."

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