According to US officials, a suspected acoustic attack on US embassy staff in Cuba was reported as recently as last month.
It was originally thought that the incidents had ended several months ago.
The State Department also said the number of staff who have reported health problems had increased to 19.
It comes as the union representing diplomatic staff says some victims suffered mild brain injuries and permanent hearing loss.
Cuba has denied any involvement in the attacks and says it is investigating the reports.
US embassy staff and at least one Canadian began to notice symptoms late last year.
However, the affair was first reported in August when the US expelled two Cuban diplomats from Washington.
US officials said the expulsions were in protest at Cuba’s failure to protect its diplomats.
Sonic devices may have been used to emit inaudible sound waves that can cause deafness.
On September 1, the US government confirmed that an incident took place as recently as August and that the number of staff reporting problems had increased.
State Department spokeswoman Heather Nauert said: “We can’t rule out new cases as medical professionals continue to evaluate members of the embassy community.”
In a statement, the American Foreign Service Association, which represents US diplomatic and international aid staff, said it had spoken to 10 people who had received treatment.
“Diagnoses include mild traumatic brain injury and permanent hearing loss, with such additional symptoms as loss of balance, severe headaches, cognitive disruption and brain swelling,” the statement said.
The American Foreign Service Association urged the government to do everything possible to help those affected and to “ensure that these incidents cease and are not repeated”.
The statement is the first time that the hearing loss has been described as permanent. It is understood that “mild traumatic brain injury” could include concussion or headaches.
The State Department is yet to blame anyone for the incidents.
The US mission in Cuba was reopened as a full embassy in 2015 following 50 years of hostilities between the two countries.