The State Department has previously said brain injuries suffered by 24 U.S. officials and family members posted in Havana could have been the result of a "health attack", perhaps from an unidentified secret weapon.
The message urges staff and family members to contact their health care provider if they experience "any unusual, unexplained physical symptoms or events, auditory or sensory phenomena, or other health concerns".
Secretary Mike Pompeo appears to be trying to avoid the uproar caused a year ago after US lawmakers and the public found out about the medical incidents in Cuba more than six months after they started happening, by some counts.
The alert comes as a United States medical team is screening more Americans who work at the consulate in Guangzhou.
"Medical professionals will continue to conduct full evaluations to determine the cause of the reported symptoms and whether the findings are consistent with those noted in previously affected government personnel or possibly completely unrelated", she said.
State Department spokeswoman Heather Nauert said Wednesday "a number of individuals" have been brought to the U.S. They are in addition to a U.S. worker in Guangzhou who was evacuated earlier, as the U.S. had already disclosed.
To date, 24 diplomats and family members who worked at the U.S. Embassy in Havana have been medically confirmed as suffering from a range of symptoms that seem to be associated with a high-pitched sound and other auditory sensations.
"The State Department received medical confirmation that a USA government employee in China suffered a medical incident consistent with what other US government personnel experienced in Havana, Cuba", the statement said, reiterating comments made last month by Secretary of State Mike Pompeo.
A USA official, who wasn't authorized to discuss the situation publicly and requested anonymity, told The Associated Press that the evacuees from China were being tested at the University of Pennsylvania, where patients from Cuba were also treated and evaluated.
He said he heard what sounded like "marbles rolling around a metal funnel" in his apartment complex, where other US state employees also called home, according to the newspaper.
"At this time, 24 US government personnel and family members who served in Cuba have been medically-confirmed as having symptoms and clinical findings similar to those noted following concussion or minor traumatic brain injury".
Canada in April additionally ordered households of diplomatic employees in Cuba to return residence after mysterious well being signs had been detected in 10 Canadians stationed on the island.
Earlier this week Secretary of State Mike Pompeo set up a health task force to develop a multi-agency response to the incidents in China and Cuba.
The U.S. first pulled out all nonessential staff in Cuba in September, followed shortly after by the expulsion of 15 Cuban diplomats from their U.S. post. The Guangzhou consulate opened months after the establishment of diplomatic relations between Beijing and Washington in 1979 and moved to its new purpose-built facility in 2013. It's also the one US diplomatic set up in China licensed to course of immigrant visas and deal with adoptions.
People working in The Canton Place complex, a few kilometers (miles) from the consulate, said Thursday they were just hearing about the incidents, reflecting a lack of coverage in China's entirely state-controlled media.
Aled Williams, a British teacher at a kindergarten said Thursday that the reports sounded "sci-fi-ish".