The United States has set up a task force to oversee the response to mystery ailments among its diplomats in Cuba and China, Secretary of State Mike Pompeo said Tuesday.
The State Department has previously said brain injuries suffered by 24 US officials and family members posted in Havana could have been the result of a "health attack," perhaps from an unidentified secret weapon.
Washington has said in the past that Cuban authorities must take responsibility for the safety of US diplomats on their soil, but the Havana government denies any knowledge of an attack.
Last month, another American employee fell ill with similar symptoms in China, reviving fears that a US rival has developed some kind of invisible acoustic or microwave device.
Announcing the task force, however, Pompeo said: "The precise nature of the injuries suffered by the affected personnel, and whether a common cause exists for all cases, has not yet been established."
According to the State Department, the "Health Incidents Response Task Force" will coordinate both the investigation and the treatment of the victims and work with several government and military agencies.
The statement makes no reference to the possibility of a deliberate attack.
But it warns that US diplomats should alert their mission's medical staff "if they note new onset of symptoms that may have begun in association with experiencing unidentified auditory sensations."
"Reported symptoms have included dizziness, headaches, tinnitus, fatigue, cognitive issues, visual problems, ear complaints and hearing loss, and difficulty sleeping," it adds.