The alert urges US citizens across China to seek medical help if they experience any “unusual, unexplained physical symptoms or events, auditory or sensory phenomena, or other health concerns”.
Symptoms include dizziness, headaches, tinnitus, fatigue, cognitive issues, visual problems, ear complaints and hearing loss, as well as difficulty sleeping, the warning said.
The unexplained health problems have already prompted the evacuation of a number of US government employees from the southern city of Guangzhou, where a US consulate is based.
In May, one worker there reported experiencing “abnormal sensations of sounds and pressure” – sparking fears of a “sonic attack”.
The state department said the symptoms indicated a “mild traumatic brain injury” and they issued a first warning covering only Guangzhou.
The US government has brought a group of people from the consulate back to the US for evaluation.
Screening has also been offered to employees at the country’s embassy in Beijing and other US consulates on request.
China has said it investigated the initial case and found no explanation.
An editorial in Beijing’s state-run Global Times newspaper said: “Practically all Chinese people do not believe that this country’s official organisations would carry out such sonic attacks against US diplomats.
“This does not fit with China’s basic concept and principles of diplomacy, and is inconceivable.”
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A similar situation led the US to recall non-essential embassy staff from Cuba in 2017, after at least 24 US citizens suffered symptoms, leading to suspicions that they had been deliberately and covertly targeted.
US investigators are looking into whether the first employee affected in China experienced a “sonic attack”, which was one of the theories behind the still-unexplained illnesses suffered in Havana.