Updated 2:51 p.m. ET, 12/10/13
(CNN) – Arriving on stage at FNB stadium in Johannesburg to pay tribute to Nelson Mandela, President Barack Obama shook hands with dozens of other world leaders, pausing briefly to grasp the hand of Cuban President Raul Castro.
The greeting quickly sparked a strong debate on Twitter between those who praised and disagreed with the handshake, given that the United States does not share diplomatic relations with Cuba.
But a senior administration official said it was not "pre-planned encounter."
"Above all else, today is about honoring Nelson Mandela, and that was the President's singular focus at the memorial service," the official continued. "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."
Nonetheless, it was a moment of high symbolism. The U.S. and Cuba have not had diplomatic relations since the Cuban Revolution more than 50 years ago. The President has eased some of the economic embargo and travel restrictions that the administration of President George W. Bush strongly enforced, but relations still are tense. Cuba continues to imprison an American citizen, Alan Gross, who was arrested in 2009 on charges of attempting to destabilize the Cuban government.
Obama knew, of course, that Castro would be on stage. But refusing to shake Castro's hand would not have been in keeping with Mandela's legacy of reconciliation.
"It took a man like Madiba to free not just the prisoner, but the jailer as well to show that you must trust others so that they may trust you; to teach that reconciliation is not a matter of ignoring a cruel past, but a means of confronting it with inclusion and generosity and truth," Obama said in his speech at the memorial service.
It was not the first handshake between American-Cuban leaders. In 2000, at the United Nations, then President Bill Clinton shook hands with Fidel Castro, the leader of the Cuban Revolution, its first revolutionary president, and Raul's brother.
While some saw Obama's handshake with Castro as nothing more than a moment of politeness, other saw it as a missed opportunity.
"If the President was going to shake his hand, he should have asked him about those basic freedoms Mandela was associated with that are denied in Cuba," Sen. Marco Rubio, a Florida Republican whose parents emigrated from Cuba, said in a statement.
Republican Rep. Ileana Ros-Lehtinen of Florida, who was born in Cuba, made her feelings known to Secretary of State John Kerry in a congressional hearing.
"Mr. Secretary sometimes a handshake is just a handshake, but when the leader of the free world shakes the bloody hand of a ruthless dictator like Raul Castro, it becomes a propaganda coup for the tyrant," she said. "Raul Castro uses that hand to sign the orders to repress and jail democracy advocates."
Kerry signaled no policy changes toward Cuba, and argued the President urged world leaders in his speech to uphold basic human rights.
Pressed by Ros-Lehtinen on whether Castro is upholding those rights, Kerry flatly answered: "No. Absolutely not."
While the President did not mention Cuba by name in his speech, some of his remarks seemed directly aimed at dictatorial regimes.
"There are too many leaders who claim solidarity with Madiba's struggle for freedom, but do not tolerate dissent from their own people," he said. "And there are too many of us on the sidelines, comfortable in complacency or cynicism when our voices must be heard."
CNN Chief National Correspondent John King recalls it was a different story at the inauguration of Mandela in 1994, when Vice President Al Gore went out of his way–ducking behind aides, through doors–to avoid a greeting with then-Cuban President Fidel Castro.
"But an inauguration is very different from a memorial service," King added on CNN's "New Day." "Raul Castro was right there. I would say the President of the United States really didn't have much of a choice."
Had he lingered a long time, King said, Obama might have started a bigger backlash than the one he'll likely receive.
"But make no doubt about it...somebody will decide that was a horrible thing," King continued. "I think the President was showing respect for the moment."
The reaction on Twitter was divided:
Obama didn't shake Castro's hand b/c Mandela "brings ppl together, even in death". He shook his hand b/c he admires & worships Communists!—
(@MaydnUSA) December 10, 2013
Easy people, it was just a handshake! It's not as if Raúl Castro just got kissed on the lips by Obama.—
Don Giovanni (@GDiangelo) December 10, 2013
Things Mandela does even after dead: Barack Obama and Raul Castro shaking hands.—
Filipe S. Henriques (@fhenriques) December 10, 2013
Obama's subtweet to Raul Castro: "Too many leaders claim solidarity with Madiba's struggle but don't tolerate dissent from their own people"—
Gavin Davis (@gavdavis) December 10, 2013
– CNN's Ashley Killough, Jim Acosta and Paul Steinhauser contributed to this report.