The new US disclosures came the same day that the union that represents American diplomats said mild traumatic brain injury was among the diagnoses given to diplomats victimised in the attacks, offering the most detailed account to date of the growing list of symptoms.
Permanent hearing loss has also been diagnosed, and the American Foreign Service Association said additional symptoms had included brain swelling, severe headaches, loss of balance and "cognitive disruption."
"We can confirm another incident which occurred last month and is now part of the investigation," Nauert said.
US officials had previously said that the attacks, initially believed to be caused by a potential covert sonic device, had started in fall 2016 and continued until spring 2017. Last week, Nauert had said at least 16 Americans associated with the US Embassy in Havana had been affected, but that the "incidents" were no longer occurring.
"We can't rule out new cases as medical professionals continue to evaluate members of the embassy community," Nauert said. She added that the embassy has a medical officer and has been consistently providing medical care to those who have reported incidents.