At least 16 US government employees have been attacked in the US embassy in Cuba, the state department says.
In an unusual case, the staff appear to be the targets of an acoustic attack. Several embassy staff have been required to seek treatment for their symptoms.
The attacks, the effects of which were first noticed late last year, appear to have stopped, said spokeswoman Heather Nauert.
Cuba has denied involvement and said it was investigating the allegations. Washington and Havana only re-established ties two years ago, following 50 years of hostilities.
The attack could be linked to sonic devices which emit inaudible soundwaves that can cause deafness, according to the Associated Press.
"We can confirm that at least 16 US government employees, members of our embassy community, have experienced some kind of symptoms," Ms Nauert said.
"We take the situation extremely seriously," she added.
Earlier this month two Cuban diplomats were expelled from Washington in response to the incident. Security around the diplomats’ residences in Havana has increased while America, Canada and Cuba search for answers.
The victims, including a Canadian, have been treated in the US and by US doctors in Cuba following the attack. A number of the diplomats have been withdrawn from Cuba.
US Secretary of State Rex Tillerson has described the incident as a "health attack".