U.S. and Cuban officials welcomed the first day of their historic diplomatic talks in Havana on Wednesday as "productive" and "constructive" despite lingering differences over migration issus.

The first of two days of talks aimed at ending decades of Cold War enmity focused on migration.

A Russian intelligence warship docked in Havana on Tuesday, a day before the start of historic U.S.-Cuba talks aimed at normalizing diplomatic relations.

There was nothing stealthy about the arrival of the Viktor Leonov CCB-175, which was moored to a pier in Old Havana where cruise ships often dock. But the visit was not officially announced by Cuban authorities.

Colombia's military denied a claim by the FARC rebel group that eight soldiers were killed this week in a counter-insurgency operation.

Military sources disclosed, however, that a soldier was killed in a separate engagement on January 9 in the southern department of Caqueta.

Cuba has freed all 53 political prisoners agreed to in last month's normalization deal with the United States, the U.S. State Department said Monday.

News of the release comes as U.S. officials prepare to travel to Havana next week for talks to lay the foundation for a historic rapprochement after a half-century of Cold War enmity.

At least 35 Cuban political prisoners have been released in the last two days, dissident leaders said Thursday, as Cuba moves towards normalizing ties with the United States.

Washington and Havana announced a historic bilateral rapprochement in mid-December in which Cuba agreed to free 53 political prisoners as part of a deal to end a five-decade standoff with the United States.

Cuba's arrest of several dissident activists caused a new rift Wednesday with the United States, the first diplomatic scuffle since this month's historic announcement of a renewal in ties.

Washington condemned Havana's "lack of respect" for human rights after the communist authorities on Tuesday arrested leading figures expected at an open mic session convened for Cubans to speak out about their future.

The United States expressed concern Wednesday over the plight of several dissidents detained by Cuba, just two weeks after Washington's historic move to normalize relations with Havana.

"We are deeply concerned about the latest reports of detentions and arrests by Cuban authorities of peaceful civil society members and activists," the U.S. State Department said in a statement.

Colombian President Juan Manuel Santos called Tuesday for serious peace talks when negotiations between his government and Marxist FARC guerrillas resume this week to ensure a "happy ending."

Santos' office said government representatives and members of the Revolutionary Armed Forces of Colombia (FARC) would resume talks in Havana on Wednesday.

Three victims of Colombia's 50-year-old conflict who gave testimony as part of peace talks in Havana have received death threats since returning home, the United Nations said Wednesday.

Of the 24 people who have so far told their stories at the talks, three "have appeared on blacklists or received mail with death threats, signed by armed groups launched by ex-paramilitaries," said Fabrizio Hochschild, the U.N.'s resident coordinator in Colombia.

Cuban dissident Guillermo Farinas said Sunday he has been placed under house arrest to keep him from taking part in an opposition forum on the sidelines of a Latin American summit.

"Today is the third day they won't let me go out," Farinas told Agence France Presse by telephone from his home in Santa Clara, 280 kilometers east of Havana.