U.S. citizens traveling in Cuba could "be at risk" of suffering from mysterious attacks that have already harmed at least 21 American diplomats, the State Department warned Friday.

"U.S. embassy personnel are most at risk but ... the American public traveling in Cuba might also be at risk as well," a senior State Department official told reporters Friday morning.

That warning accompanied the most detailed State Department description of the symptoms of the attacks, which range from "hearing loss" to "visual complaints" and other "cognitive issues, according to the official.

The warning came after Secretary of State Rex Tillerson decided to withdraw most U.S. personnel from Cuba, and a decision that the Trump administration will curtail engagement with the Castro regime in the country, although American investigators will continue their work in Havana.

"The governments of the United States and Cuba have not yet identified the responsible party, but the government of Cuba is responsible for taking all appropriate steps to prevent attacks on our diplomatic personnel in Cuba," the official said.

The attacks undermine efforts to implement former President Obama's decision to normalize relations with Cuba, a decision that President Trump decided not to reverse. Still, the curtailing of most U.S. government travel to Cuba and the curtailing of American tourism to the island country is a blow to the project.

Tillerson's team described the attacks as "ongoing," although the last reported incident was in August. At least some of the attacks took place at "hotels where American citizens could be at," which prompted the travel warning.

"We have no way of advising American citizens on how they could mitigate such attacks, [and so] we felt that we must warn them not to travel to Cuba," the State Department official said.

U.S. investigators have not ruled out the possibility that a third-party country perpetrated the attacks in Cuba. "That investigation continues and it will continue irrespective of the ordered departure," the official said.

That State Department stopped short of closing the embassy in Havana and expelling the Cuban diplomats in the United States, as some lawmakers have urged Tillerson to do. Instead, the embassy will be manned by emergency staff available for essential activities, "including providing emergency assistance to U.S. citizens in Cuba."

But they will not process "routine visa operations" and most U.S. government personnel, diplomatic or otherwise, will not be traveling to Cuba.

"Meetings may continue in the United States," the official said. "We maintain diplomatic relations with Cuba and our work in Cuba continues to be guided by the national security and foreign policy interests of the United States."