The White House on Wednesday dismissed Republican criticism of President Barack Obama's handshake with Cuban leader Raul Castro at Nelson Mandela's memorial service.

Obama has faced a backlash from Republican rivals after exchanging pleasantries with Castro, the leader of a Communist-run country that has been bitterly estranged from the United States since 1961.

The White House has insisted the gesture was not "pre-planned" while a Cuban government-run website noted hopefully that the handshake could "be the beginning of the end of the US aggressions against Cuba."

White House spokesman Josh Earnest played down the significance of the exchange on Wednesday, stating it had been informal.

"It's my understanding, based on people who did talk to the President after his speech, that they didn't have a robust, substantive conversation about policies, but rather exchanged some pleasantries as the President was making his way to the podium," Earnest told reporters.

"So there was not an opportunity for the president to chronicle his many concerns about human rights abuses on the island of Cuba," he added.

The case of US subcontractor businessman Alan Gross, jailed in Cuba for 15 years in 2011, was not mentioned, Earnest said.

Republicans had likened Obama's gesture to the appeasement of the Nazis, in a chorus of criticism.

Republican Senator John McCain said Obama was wrong to "shake hands with somebody who is keeping Americans in prison."

"What's the point?" McCain said. "Neville Chamberlain shook hands with Hitler."

Earnest hit back at the comparison, describing it as "dangerous and usually unwise."

"There used to be a pretty important principle, that originated in the Republican Party, I believe, that partisan politics should stop at the water's edge," Earnest said.

"It's unfortunate that we did see a number of Republicans yesterday who criticized the president for a handshake at Nelson Mandela's funeral."