When U.S. President Obama and Cuban President Raul Castro stepped up to a podium Monday in Havana to mark the first visit by a U.S. president there in 88 years, the event came with a remarkably sharp exchange on several issues. Among them: It’s time, Castro said, for Washington to give back the “illegally occupied” naval base at Guantanamo Bay back to Cuba.

Castro characterized the U.S.’s control of Guantanamo as one of the “two main obstacles” to the United States and Cuba normalizing its relations. The other, he said, is the financial, economic and commercial embargo that Washington has long had on Cuba.

While the Obama administration has shown some willingness to opening trade relations with Cuba, it has “no intention at present” to alter the lease that gives it control of the naval base at Guantanamo, a senior administration official said Monday after Castro spoke. The official, who spoke on condition of anonymity in order to speak candidly about diplomatic relations, reaffirmed the president’s desire to close down the military prison on the base, but said the naval base has a large and varied mission that stretches well beyond the detention facility.

“The administration is determined to close the Guantanamo Bay detention facility,” the official said. “The continued operation of the detention facility weakens our national security by draining resources, damaging out relationships with key allies and partners, and emboldening violent extremists.  We are taking all possible steps to reduce the detainee population at Guantanamo and to close the detention facility in a responsible manner that protects our national security.”

Another U.S. official said the administration has consistently said giving back Guantanamo is not in its plans.

The United States has maintained the base, which lies on Cuba’s southeastern tip, since 1903. Cuba has long protested its presence, saying the land was taken by force during the U.S. invasion of Cuba at the turn of the 19th century. But a treaty signed between the two nations in 1903 and reaffirmed in 1934 states that the United States has control of Guantanamo Bay unless it vacates it or strikes a deal with Cuba that says otherwise.

“So long as the United States of America shall not abandon the said naval station of Guantanamo or the two Governments shall not agree to a modification of its present limits, the station shall continue to have the territorial area that it now has,” the treaty states.

Following Fidel Castro’s rise to power in the 1950s, the naval station at Guantanamo became increasingly isolated. According to the U.S. Navy’s history of the station, Castro cut off water to the base in 1964, and since then Guantanamo has been completely self-sufficient in regards to both water and power. The perimeter of the base is littered with Cuban mines and U.S. ground sensors. American mines also were once part of the security for the base, but President Clinton ordered them removed in the 1990s. The guard posts are manned by U.S. Marines — something that was popularly depicted in the 1992 movie “A Few Good Men.”

In the early 1990s, the base assisted in a numerous humanitarian relief mission that involved both Haitian and Chinese refugees. The naval station has a number of tenets currently, including a communications station and the Naval Atlantic Meteorology and Oceanography Command.

The base’s most recent addition is Southern Command Joint Task Force-Guantanamo, the unit that runs the military prison. It was once a myriad of different lettered camps that were rolled into one in the months after the Sept. 11 attacks.

Read more: