PANAMA CITY - US President Barack Obama and Cuba's Raul Castro will break bread with other leaders from around the Americas at a historic summit Friday, a potent symbol of their efforts to end decades of animosity.

As the gathering in Panama approached, a White House official revealed that Obama and Castro had spoken by telephone Wednesday - just the second phone call between a US and Cuban leader in more than 50 years, and the first since December, just before they announced the game-changing diplomatic thaw.

The presidents will join some 30 other leaders at the two-day Summit of the Americas, posing for pictures and sitting down for a seaside dinner in a complex of ruins from the era of the Spanish conquistadors.

Obama began his day with a visit to the Panama Canal, where, jacket slung over his shoulder in the tropical heat, he toured a command center and walked across the Miraflores locks, at the Pacific entrance to the interoceanic waterway.

The US and Cuban chief diplomats, Secretary of State John Kerry and Foreign Minister Bruno Rodriguez, made history themselves Thursday evening when they held talks - the first such meeting since 1958, a year before Fidel Castro's revolutionary guerrillas seized power.

Both sides said the nearly three-hour talks were "constructive" and would be followed by further conversations to resolve outstanding issues.

While the meetings are packed with powerful symbolism, the two countries have a long road ahead in their broader goal of normalizing relations.

An Obama-Castro meeting is "part of the overall negotiations that are taking place," said former Cuban diplomat and foreign relations professor Carlos Alzugaray. "This doesn't end with Raul's presence at the summit, it's the beginning."

Obama acknowledged as much on Thursday during a visit to Jamaica, before landing in Panama. "I never foresaw that immediately overnight everything would transform itself, that suddenly Cuba became a partner diplomatically with us the way Jamaica is, for example," he said. "That's going to take some time."

The US leader may bring to the table a resolution to an old gripe from Cuba, as a senator said the US State Department recommended that Obama remove Havana from a list of state sponsors of terrorism.

Cuba's status on the blacklist - which also includes Iran, Syria and Sudan - has been a major sticking point in negotiations to reopen embassies that closed after relations broke in 1961. Cuba was first put on the list in 1982 for harboring Basque separatist militants and Colombian FARC rebels, restricting the country's access to global bank credit.

Removing Cuba from the terror-sponsor list would not be immediate. Congress would have 45 days to decide whether to override Obama's recommendation.

US lawmakers who have been critical of the diplomatic detente could seize on the review of the list to further attack Obama's Cuba policy. Cuba has other major demands, most importantly that the US Congress lift an embargo that the communist regime blames for the island's economic troubles.

Washington wants Cuba to lift restrictions on the movement of its diplomats on the island, giving them unfettered access to ordinary Cubans. The reconciliation appears popular in both countries.

A Marist College poll showed this week that 59 percent of Americans back the diplomatic thaw, while a survey by US pollster Bendixen & Amandi International in Cuba found that 97 percent of islanders are in favor. But Cuban government supporters confronted dissidents on the sidelines of the summit, heckling them as they attended a civil society forum.

A meeting between Castro and Obama at the summit will provide a photo opportunity to immortalize the diplomatic reconciliation. The two leaders briefly shook hands at Nelson Mandela's funeral in 2013, but they now have a chance for more face time.

The White House said the two would have time to interact, but the extent of the encounter remains a mystery and could fall short of a formal, bilateral meeting.

But as Obama moves to remove an old source of tension in US relations with Latin America, a new headache has emerged since he imposed sanctions against Venezuelan officials accused of human rights abuses in an opposition crackdown.

Venezuelan President Nicolas Maduro, Havana's main ally in the region, said Thursday he had gathered 13.4 million signatures in a petition urging Obama to lift his executive order, which calls Caracas a US national security threat.

Maduro welcomed White House statements saying it does not see Venezuela as a threat. But, he added, "I ask Obama why he signed this order. If he doesn't answer... it will be impossible to open a new era" in US-Venezuelan relations.