HAVANA (Reuters) - The United States said on Friday it was making permanent its decision last year to slash staffing at its Havana embassy by around two-thirds because a spate of alleged health incidents among its diplomats remained unsolved.

A vintage car drives in front of the U.S. Embassy in Havana, Cuba, March 2, 2018. REUTERS/Stringer

The decision casts a pall on U.S.-Cuban relations and limits the United States’ ability to play a role in the civil society of the Communist-run island as it prepares for its first non-Castro president in nearly 60 years.

It will also hurt Cuban-American families divided by the Florida Straits, who have struggled to get visas to visit one another since the United States first cut the embassy’s staff in September due to the “attacks” that it says have caused hearing loss, dizziness and fatigue in two dozen employees.

President Donald Trump’s administration also expelled 17 Cuban diplomats stationed in the United States.

By law, the State Department is required to decide whether to send diplomats back six months after an ordered departure, and the deadline for that decision was up this weekend.

“The embassy will continue to operate with the minimum personnel necessary to perform core diplomatic and consular functions, similar to the level of emergency staffing maintained during ordered departure,” the State Department said in a statement.

The embassy would now operate as an unaccompanied post, it said, meaning diplomats would not be permitted to move there with family members.

The department also renewed its Cuba travel warning on Friday, saying U.S. travelers could be at risk from the mysterious “health attacks,” dealing a blow both to both U.S. travel companies and the island’s fledgling private sector, which had benefited from a boom in U.S. visits.

Cuba and a number of analysts have said Trump’s Republican administration was using the alleged health incidents to justify unwinding a detente begun in 2014 by Democratic former U.S. President Barack Obama and Cuban President Raul Castro.

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Cuba, which is conducting its own investigation, says there is no evidence of any attacks. The United States has not accused Cuba of perpetrating them, but holds it responsible for ensuring the safety of it diplomats.

On Friday, the State Department said it still had no “definitive answers on the source or cause of the attacks” and a probe was ongoing.

“U.S.-Cuban relations have become trapped in a geopolitical quagmire over the embassy health mystery,” said Cuba analyst Peter Kornbluh.

“After a year of intense investigation on both sides that has failed to identify what has caused the health problems, it is hard to see how this impasse will be resolved.”

Trump has taken a tough stance on Cuba, tightening trade and ordering travel restrictions that harken back to the Cold War era just as Cuba embarks on a major political transition.

Raul Castro is due to step down as president in April. Meanwhile, major economic policy changes are expected this year, such as the unification of Cuba’s multiple exchange rates and a possible course adjustment in market reforms.

“We have lost the strategic opportunity to pull Cuba into our sphere of interest,” said Vicki Huddleston, a former head of the U.S. interests section in Havana.

“Cuba always needs to have benefactor ... now the next benefactor will likely be Russia or China.”

The U.S. embassy has typically maintained close ties with civil society and the opposition in Cuba. Yet under current staffing levels - the lowest since the U.S. Interest Section opened in 1977, according to Cuba experts - it does not even have a human rights officer.

The embassy halted regular visa operations last year and is only offering emergency services to U.S. citizens. Cubans seeking visas to the United States must apply at U.S. embassies abroad, which is costlier and more complicated.