US President Barack Obama's cordial handshake with Cuban leader Raul Castro at Nelson Mandela's memorial service in South Africa was not "pre-planned" and was just an exchange of greetings, the White House has said.

"Nothing was planned in terms of the President's role other than his remarks. So the rest of it, of course, was simply his presence at the ceremony," Deputy National Security Advisor Ben Rhodes told reporters aboard Air Force One on the way back to Washington from the service in Johannesburg.

"This wasn't a pre-planned encounter," a White House official said. "Above all else, today is about honouring Nelson Mandela, and that was the president's singular focus at the memorial service. We appreciate that people from all over the world are participating in this ceremony," the official said yesterday.

Obama knew that he was exchanging greetings with Castro, Rhodes said. "Yes, he was aware that that was Raul Castro."

"I think it was just an exchange of greetings as he was making his way through the podium up to speak," he said, adding that Obama's policy towards Cuba remains the same.

"Look, as a general matter, we have taken a different approach in some important respects in relation to Cuba. The President lifted the prohibition on family travel and remittances. He increased licensing for travel and engagement with Cuba," he said.