HAVANA - A half-century after Washington severed relations with Cuba, the United States' seven-story mission looms over Havana's seaside Malecon boulevard as the largest diplomatic outpost in the country.

Cuban guards stand at close intervals on the street outside, and islanders line up by the thousands each year for a shot at a coveted visa.

The gleaming U.S. Interests Section suddenly is poised to become an even more important presence in Cuba as the two countries negotiate the first phase of their historic détente - transforming the complex into a full embassy that would reflect the Obama administration's hopes of new influence on the communist island.

Roberta Jacobson, assistant U.S. secretary of state for Western Hemisphere affairs, will be the highest known U.S. administration official to visit in decades when she comes next month for annual talks on migration that will now also focus on the details of reestablishing full diplomatic relations.

The discussions are expected to cover expanding staffing in the two countries' interests sections and letting diplomats travel outside their respective capitals without having to ask permission.

Also part of the reopening of the embassy: symbolic measures such as raising the American flag on the Malecon.

"Opening an embassy is a symbolic gesture, but symbols are really important," John Caulfield, Interests Section chief from 2011 to 2014, said by phone from Jacksonville, Fla., where he retired.

"This is a pretty powerful symbol by our president," he added, "that we want to have a more normal relationship with Cuba despite the fact that we have the obvious differences."

Cuba's interests section is a stately manor in Washington's Adams Morgan neighborhood. It, too, stands to become an embassy.

Diplomats say privately that Washington hopes to boost staffing in Havana, currently about 50 Americans and 300 Cuban workers, as more American travelers and trade delegates are expected to come here.

The U.S. Interests Section building first opened as an embassy in 1953, the same year Fidel Castro launched an ill-fated assault on a barracks, considered the onset of the Cuban Revolution.

Eight years later, with Castro then in power, the countries broke ties, and Switzerland stepped in to safeguard both the embassy and the ambassador's residence.

After the break, Washington was without a presence in Cuba until 1977, when the interests sections were opened under President Jimmy Carter.

The U.S. Interests Section is closely watched by cameras and guards on both sides, a function of both the longtime tensions and general increased security at American diplomatic missions after 9/11.

Both sides gradually moved toward a remarkably civil relationship in recent years. Some who served in the U.S. Interests Section are awaiting the change with a mixture of excitement and wistfulness at not being here for the transition.

"I would have loved to be there," Caulfield said, "to raise that flag."