It follows similar incidents in Cuba last year that affected at least 24 people
A special task force has been set up in the US to respond to 'unexplained health incidents' affecting a number of Americans working overseas and their family members.
It comes as officials investigate a new case affecting at least one American working in China.
Last year, reports of 'unexplained hearing loss' and other health issues emerged from the newly reopened US embassy in Havana.
According to the latest update from the US State Department, at least 24 US government employees or family members in Cuba have experienced illnesses similar to "concussion or minor traumatic brain injury".
Symptoms have included headaches, hearing loss, dizziness and cognitive issues.
Some reports suggested 'sonic harassment attacks' could be responsible for the mystery illnesses, but no cause has been officially confirmed.
Medical tests have now confirmed that one US government employee serving in China has experienced similar symptoms to those experienced by the Cuban diplomats.
While the Trump administration has confirmed that case, the New York Times reports that at least two Americans who worked at the US consulate in the Chinese city of Guangzhou have been evacuated.
Other staff and family members are now being tested for symptoms in the wake of the incident.
In a statement quoted by Reuters, State Department spokesperson Heather Nauert said "a number of individuals" have been sent to the US for "further evaluation and a comprehensive assessment of their symptoms and findings".
She added that officials will investigate whether any cases from China are consistent with the Cuba ones or "completely unrelated".
Separately, US Secretary of State Mike Pompeo said: "At my request, [officials] established a task force on May 23 to direct a multi-agency response to the unexplained health incidents that have affected a number of US government personnel and family members stationed overseas.
"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."
When news of the Cuba mystery first emerged, the Cuban government insisted it would never allow its territory "be used for any action against accredited diplomatic agents or their families".