State Department spokesperson Heather Nauert linked "incidents which have caused a variety of physical symptoms" to the decision to expel two Cubans, but she did not directly blame the Cuban government for harming the Americans.
'We do not have definitive answers on the source or cause of the incidents'.
A senior federal law enforcement official tells NBC News that several U.S agencies are investigating allegations that the Cubans directed some kind of ultrasound energy at US diplomats which left several with damaged hearing. She did not describe the nature or cause of the symptoms, but said they started in 2016.
Officials, speaking under condition of anonymity, said an investigation revealed that the diplomats had been affected by a device emitting noises outside the range of audible sound.
The US says two Cuban diplomats have been expelled in response to "incidents" in Cuba, but the details remain mysterious.
One official told the Associated Press news agency that it could be potentially permanent hearing loss.
Towards the end of past year, USA diplomats began to suffer mysterious hearing loss, according to investigators.
Cuba's foreign ministry said it was investigating the allegations.
Investigators are exploring the possibility that another country, such as Russian Federation, carried out the attack without the Cuban government's knowledge.
The Ministry of Foreign Affairs reaffirms that Cuba complies rigorously and seriously with all its obligations arising from the 1961 Vienna Convention on Diplomatic Relations in connection with the protection and inviolability of diplomatic agents and the premises of diplomatic missions.
The Cuban government itself released a statement on Wednesday in which it denied any wrongdoing and said the expulsion of its diplomats was "unjustified and baseless". "We take those incidents very seriously and there is an investigation now underway".
"It is a cause of great concern for us", she said.
"The Cuban authorities established an interagency committee of experts for the analysis of the facts; extended and strengthened security and safety measures for headquarters, staff and diplomatic residences, and enabled new channels of direct communication between the Embassy and the Department of Diplomatic Security", the statement from the Cuban Foreign Ministry said.
President Donald Trump, who made it clear he opposed the Obama administration's "détente" with the Castro regime during his 2016 campaign, had already scaled back many of his predecessor's diplomatic overtures. CNN quotes an unnamed official who stated, "We have worked with the Cubans to try and find out what is going on".
The U.S. Embassy building in Havana, shown in 2015.
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