Editor's Note: The Axe Files, featuring David Axelrod, is a podcast distributed by CNN and produced at the University of Chicago Institute of Politics. The author works at the institute.

Havana (CNN) In the wake of the Orlando nightclub massacre last month, President Barack Obama received an unexpected letter of condolence. It was from Cuban President Raul Castro.

Such a gesture would have been unthinkable even a year ago. But Jeffrey DeLaurentis, America's top diplomat in Cuba, says it is yet another sign of the thawing relations between the U.S. and the island national in the wake of the president's visit there in March.

He made the comments to David Axelrod on "The Axe Files" podcast, produced by CNN and the University of Chicago Institute of Politics.

DeLaurentis, chief of mission at the U.S. Embassy in Cuba, also called on Congress to end the half century-old economic embargo on the island nation as historic steps toward normalization of relations continue to proceed.

"The President has called for the embargo to be lifted... Going back to the initial idea of engagement, it's expanding people to people, expanding the flow of information, and expanding commercial contact, this fits in very much with that to provide opportunities here and there. So we think it's important that the embargo is lifted," he said.

DeLaurentis, who has served as charge d'affaires to Cuba since August 2014, said the two countries are continuing to negotiate additional normalization measures that can be achieved through the executive authority of the president.

"We've set up, I would say, a very aggressive agenda for the remaining part of the administration," he said. "We'll meet a couple more times, so I'm optimistic about that, in terms of the growing areas where we can cooperate to the benefit of both sides. And I think there's an effort by both sides to try and make as much progress as we can, and that to me is a sign of optimism."

US Charge d'Affaires in Cuba Jeffrey DeLaurentis introduces President Obama to embassy staff in March.

"Toward the end of the administration it becomes more and more difficult to make these kinds of changes as we get closer. But depending on how things go, there may be a possibility for... either ways to perfect the changes that we have made already and perhaps explore some others."

But DeLaurentis said the U.S. would continue to prod Cuba to improve its record on human rights.

"For us, the protection and promotion of civil and political rights, these are universal values," he said. "This is something that we believe all peoples should be able to enjoy and express. We make these views known everywhere we go, we'll continue to advocate for them, and we believe with this normalization process -- with the establishment of diplomatic relations -- we now have mechanisms where we can articulate those and advocate for those directly and to the right folks and publicly, sometimes, when we have to."