Cuba has said it is investigating allegations by the United States that several diplomats at the US embassy in Havana developed hearing problems last year which forced them to return home.
It has been suggested that covert sonic devices which emit inaudible sound waves may have been placed either inside or outside the diplomats residences in the Cuban capital.
Officials in Washington have confirmed that staff suffered a variety of physical symptoms but it is unclear whether there was any deliberate intention to cause harm to the diplomats.
The US has also since expelled two Washington-based Cuban diplomats.
The Cuban foreign ministry has denied any involvement by the government in Havana.
"Cuba has never, nor would it ever, allow that the Cuban territory be used for any action against accredited diplomatic agents or their families," the foreign ministry said in a statement. "It reiterates its willingness to cooperate in the clarification of this situation."
The foreign ministry said it had started a "comprehensive, priority and urgent investigation" into the alleged incidents after it had been informed of them by the US embassy in February.
Yesterday, US State Department spokeswoman Heather Nauert told reporters that the exact nature of the incidents was unclear, but Americans serving in Cuba had returned to the US for non life-threatening "medical reasons."
The US first learned of the issues at the embassy in late 2016, she said.
"We don't have any definitive answers about the source or the cause of what we consider to be incidents," Ms Nauert said.
"It's caused a variety of physical symptoms in these American citizens who work for the US government. We take those incidents very seriously, and there is an investigation currently under way."
As a result, the US asked two Cuban officials on 23 May in Washington to leave the country and they have done so, Ms Nauert said, an action that Cuba described as "unjustified".
"What this requires is providing medical examinations to these people," Ms Nauert said. "Initially, when they'd started reporting what I will just call symptoms, it took time to figure out what it was, and this is still ongoing. So we're monitoring it."
A US government official said several colleagues at the US embassy in Havana were evacuated back to the US for hearing problems and other symptoms over the past six months.
Some subsequently got hearing aids, said the official,who spoke on condition of anonymity.
Relations between the US and Cuba were restored in 2015 after a half-century break by then US president Barack Obama and his Cuban counterpart Raul Castro.
But relationships were partially rolled back by President Donald Trump who in June announced tightened rules for Americans traveling to Cuba, banned ties with a military-run tourism firm and reaffirmed the existing US trade embargo.