The U.S. Embassy in China issued a health alert Wednesday to all government workers after an employee showed signs of a brain injury after hearing "abnormal" sounds and feeling unusual physical pressure.
The Canadian government in April said it would remove families of diplomats posted to Cuba after Canadian personnel there in 2017 also reported similar health symptoms.
Wednesday's alert said the US government does not know the cause of the reported symptoms and has not received similar reports in other parts of China, but that it is taking the report seriously.
The US Embassy in Beijing learned on May 18 that the clinical findings of the evaluation matched that of a "mild traumatic brain injury", an embassy spokeswoman told CNN.
Of the 21 medically confirmed USA victims, some have permanent hearing loss or concussions, while others suffered nausea, headaches and ear-ringing.
The employee experienced the symptoms from late 2017 through April 2018 while on assignment in the southern Chinese city of Guangzhou, where a USA consulate is located, according to Jinnie Lee, a spokeswoman at the US embassy in Beijing.
The worker was sent to the United States for further evaluation.
The State Department said in its Wednesday statement that anyone who experienced "unusual acute auditory or sensory phenomena" while in China should move away from the source of the noise.
China's Foreign Ministry and National Health Commission did not immediately respond to faxed questions about the report.
The State Department was taking the incident very seriously and working to determine the cause and impact, the embassy said.
"The Chinese government has assured us they are also investigating and taking appropriate measures", the spokeswoman said. Symptoms included headaches and hearing loss. Officials believe some type of sonic weapon may have been used.
The US has held Cuba responsible, either for allowing the suspected attacks to happen or for carrying them out itself. The cause of those incidents, reported in late 2016 and early 2017, still remains a mystery.
Canadian and U.S. authorities had initially suspected a "sonic attack" or a "mass psychosomatic incident", but those are "now considered unlikely", a senior Canadian official said.