US President Barack Obama declared his refusal to refight the Cold War battles of the past while Cuban President Raul Castro rallied to his defence, absolving Obama of fault for the U. blockade in a stunning reversal of more than 50 years of animosity between the two countries.

Castro, in an hour-long speech to the Summit of the Americas in Panama City, ran through an exhaustive history of perceived Cuban grievances against the US dating back more than a century - a vivid display of how raw passions remain over American attempts to undermine Cuba's government.

Then, in an abrupt about face, he apologised for letting his emotions get the best of him. He said many US presidents were at fault for that troubled history - but that Obama isn't one of them.

"I have told President Obama that I get very emotional talking about the revolution," Castro said through a translator, noting that Obama wasn't even born when the US began sanctioning the island nation.

"I apologise to him because President Obama had no responsibility for this."

In a remarkable vote of confidence from a Cuban leader, Castro added: "In my opinion, President Obama is an honest man."

Castro and Obama were expected to meet later tonight on the sidelines of the summit - the first substantial meeting between a US and Cuban president in more than five decades. The flurry of diplomacy was aimed at injecting fresh momentum into their previously announced plan to restore normal relations between the US and Cuba.

Speaking just before Castro, Obama acknowledged that deep differences between their countries would persist. Yet he said he was uninterested in getting bogged down in ideology, instead casting the thaw in relations as an opening to create "more opportunities and resources for the Cuban people".

"The United States will not be imprisoned by the past," President Barack Obama said. "We're looking to the future."

Raising the stakes even higher for the two leaders was mounting speculation that Obama would use the occasion of the summit taking place in Panama to announce his decision to remove Cuba from the US list of state sponsors of terrorism, a gesture that for Cuba holds both practical and symbolic value.

The US long ago stopped accusing Cuba of conducting terrorism, and Obama has signalled that he's ready to take Cuba off the list. On Thursday, he suggested an announcement was imminent when he revealed that the State Department had completed its lengthy review of the designation.

Obama arrived at the summit for a day of marathon meetings with leaders from across the Western Hemisphere, gathered around a massive oval table with two birds of peace in the middle.

A successful relaunch of US-Cuba relations would form a cornerstone of Obama's foreign policy legacy. But it is an endeavour he cannott undertake alone: Only Congress can fully lift the onerous US sanctions regime on Cuba, and there are deep pockets of opposition in the US to taking that step.