Cuban and US authorities have confirmed investigations are underway amid reports that a number of US diplomats suffered serious hearing loss while stationed in Havana.
The US says two Cuban diplomats have been expelled in response to 'incidents' in Cuba, but the details remain mysterious.
The US embassy in Havana re-opened in August 2015 as part of improved diplomatic relations between the two states under Barack Obama.
The Associated Press claims that in Autumn 2016 a number of embassy staff in the Cuban capital were left with "severe hearing loss attributed to a covert sonic device".
Some staff are said to have had such severe symptoms that they were forced to return to the US.
According to the AP report - which cites anonymous US officials - it is believed that the affected staff had been "exposed to an advanced device that operated outside the range of audible sound".
The report adds that it was unclear how the staff were exposed to the device - including whether it was a deliberate attack.
"A variety of physical symptoms"
US State Department spokesperson Heather Nauert confirmed to reporters that it was investigating "incidents which have caused a variety of physical symptoms" to embassy staff in Havana, but did not give any indication about the nature of the incidents or symptoms.
She said: "We don’t have any definitive answers about the source or the cause of what we consider to be incidents.
"We can tell you that on May 23rd, the State Department took further action. We asked two officials who were accredited at the Embassy of Cuba in the United States to depart the United States."
In a statement, the Cuban foreign affairs ministry said it launched an investigation after it was informed of the situation by US officials in February.
Cuban officials say they approached the matter with the "utmost seriousness", although also said the US decision to expel two Cuban diplomats was "unjustified and unsubstantiated".
The ministry insisted: "Cuba has never, nor would it ever, allow that the Cuban territory be used for any action against accredited diplomatic agents or their families, without exception."
