In a briefing with reporters, State Department spokeswoman Heather Nauert would only refer to "physical symptoms" and an "incident" at the embassy in Havana.
A spokesperson for the State Department's Bureau of Western Hemisphere Affairs says the department does not have "definitive answers on the source or cause of the incidents".
In Cuba, an acoustic attack may be to blame for severe hearing loss among U.S diplomats past year, though officials are uncertain of the cause the injuries.
One State Department official confirms reports that these "incidents" involved symptoms of hearing loss.
Some staff are said to have had such severe symptoms that they were forced to return to the US.
"The Cuban government has been harassing U.S. personnel working in Havana for decades", he said.
United States investigators have concluded that the country's diplomats were being hit with an advanced device that can send out sound that is inaudible but can cause huge damage to the ears of a person around them, which appears to have been left either inside or near the diplomats' houses.
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The US says two Cuban diplomats have been expelled in response to "incidents" in Cuba, but the details remain mysterious.
The AP said at least one of the diplomatic personnel reported hearing loss.
Meanwhile, on Fox News Channel's Special Report, Anchor Bret Baier delivered a news brief on the developing story. The statement also said it had launched an "exhaustive, high-priority, urgent investigation at the behest of the highest level of the Cuban government".
Investigators are exploring the possibility that another country, such as Russian Federation, carried out the attack without the Cuban government's knowledge. "Under the Vienna Convention, Cuba has an obligation to take measures to protect diplomats". Personal harm to US officials shows the extent the Castro regime will go and clearly violates global norms.
"Because there are a variety of symptoms, there could be a variety of sources", she said.
"Cuba has never, nor would ever, allow the Cuban territory to be used for any kind of action against accredited diplomats or their families", the foreign ministry said in a statement. "We take those incidents very seriously and there is an investigation now underway". On May 23rd, two Cuban diplomats were asked to leave the United States in an apparent response to American diplomats being forced to come home for medical reasons.