(Reuters) - Cuba released three detainees on Wednesday in a sign it was fulfilling an agreement to free a list of 53 people the United States regards as political prisoners as part of a thaw in relations between the two nations.

The government freed 19-year-old twin brothers Diango Vargas Martin and Bianco Vargas Martin on Wednesday afternoon, opposition activists said. The pair were arrested in December 2012 and sentenced to 30 months in jail for disorderly conduct and threatening a state official.

Dissidents said a third man, Enrique Figuerola Miranda, was also freed. All three were members of the opposition Patriotic Union of Cuba (UNPACU).

The move followed a report on Wednesday by Reuters citing a congressional aide who said that Cuba was resisting the release of "several" of the prisoners on the list.

A White House official had denied the Reuters report, saying "we have not heard any such thing from the Cubans, we fully expect all 53 to be released."

The prisoner release was part of an historic deal last month to renew diplomatic relations between the United States and Cuba and end more than five decades of hostility.

However, three weeks after the bid for U.S.-Cuba d├ętente became public, President Barack Obama's administration has done little to dispel questions about what might be holding up full implementation of the prisoner agreement.

The Cuban government had no immediate comment Wednesday on the status of the prisoner release.

The twins were released in the eastern city of Santiago and returned home to their mother, according to Elizardo Sanchez, president of the Cuban Commission for Human Rights and National Reconciliation.

The brothers were included on an informal list drawn up by Sanchez, but it is not known officially if they were on the list of 53 names.

"It appears that the big release has begun," said Sanchez, whose opposition group monitors the arrests of Cuban government critics. "We hope dozens more will be freed in the coming days."

Amnesty International had described the twins -- an older brother, Alexei Vargas Martin, who remains in custody -- as "prisoners of conscience" whom it says were detained solely for peacefully exercising their right to freedom of expression.

Their mother, Miraida Martin Calderin, is a member of another dissident group, the Ladies in White.

"I am a little emotional, but more than anything proud that they held firm until now against the political persecution," she told Reuters by telephone.

The third person freed, Enrique Figuerola Miranda, was arrested in 2012 and sentenced to three years, dissidents said, adding that he went on several hunger strikes behind bars.

Information about those to be freed, including names and the timetable for release, has been withheld by both governments, providing ammunition for Republican congressional opponents and other hardline critics of the policy shift.

White House spokesman Josh Earnest told reporters that one reason the prisoners were not being identified for now was because "we don't want to put an even bigger target on their back as political dissidents."

State Department spokeswoman Jen Psaki reiterated on Wednesday morning that "some" of the 53 prisoners had already been released and said Washington had been in touch with Havana to ensure the rest are freed. She declined to provide further details.

Asked on Wednesday whether Washington believed Havana was holding to its pledge on the prisoner release, Psaki told reporters: "We have not heard anything different from the Cuban government, period."

The United States and Cuba are due to talks in Havana later this month on migration and normalization of diplomatic relations.

The White House official said the two countries had agreed on a common list before the Dec. 17 announcement of an agreement to restore relations.

Despite that, the congressional aide, also speaking on condition of anonymity, suggested a possible obstacle to the release of everyone on the list. "The government in Havana believes that the smaller group has committed acts of violence," the aide said.

Florida Senator Marco Rubio, a Cuban-American Republican who is a leading congressional critic of Obama's policy shift, on Tuesday wrote to Obama to urge him to cancel the upcoming talks -- at least until all the prisoners are released.