WASHINGTON (Reuters) - A “handful” of private U.S. citizens who traveled to Cuba say they have experienced symptoms similar to those suffered by American diplomats in mysterious health “attacks” in Havana, the U.S. State Department said on Friday.

A State Department spokesperson, who declined to be named, said the agency could not verify the claims but said travelers should heed its travel warning issued last Friday.

The warning urged Americans to stay away from Cuba because of unexplained health “attacks” it says have caused hearing loss, dizziness, fatigue and cognitive issues among at least 22 diplomatic personnel.

The Trump administration on Tuesday expelled 15 Cuban diplomats to protest Cuba’s failure to protect staff at the U.S. embassy in the communist country, just days after Washington recalled more than half the U.S. diplomatic personnel from Havana.

Cuba has denied involvement, and Washington has not directly blamed the government in Havana. So far, no probes have yielded any answers about how the alleged attacks were carried out or who was responsible.

The warning said the attacks had occurred in “diplomatic residences and hotels frequented by U.S. citizens.” CBS News first reported that some private citizens had complained of symptoms after visiting Cuba.

On Friday, the U.S. embassy in Havana identified the Hotel Nacional and Hotel Capri as the two places where it said embassy personnel had been targeted over the past few months, and said the U.S. government had “imposed limitations on lodging” there.

Receptionists at both hotels, when contacted by Reuters, said they had not heard of any restrictions. Both said they had Americans registered there and their managers were not immediately available for comment.

U.S. intelligence operatives working undercover were among the embassy personnel affected by the attacks, but it was unclear if they were specifically targeted since the symptoms hit staff across a range of job categories, U.S. officials have told Reuters.

The steps taken by Republican President Donald Trump’s administration deliver another blow to his Democratic predecessor Barack Obama’s policy of rapprochement, including actions likely to erode the normalization of a relationship dominated for decades by Cold War hostility and suspicion.

Cuban Foreign Minister Bruno Rodriguez denounced the expulsion of Cuban diplomats as “unjustified,” accused the United States of insufficient cooperation with Cuba’s investigation and urged Washington to stop politicizing the matter.

Theories about the attacks abound, from surveillance technology gone awry to a sophisticated acoustic weapon in the hands of Cuban-American exiles or third-party state actors such as Russia, Iran or North Korea. But no clear explanation has emerged.