(CNN) - The US State Department on Friday issued an expanded health alert for China over concerns for the safety of US personnel in the country following reports of a series of mysterious acoustic incidents.
A health alert was first issued for Guangzhou on May 23 after a US employee stationed in the southern Chinese city was diagnosed with a mild brain injury after complaining of unexplained sonic phenomena. The latest alert is countrywide.
On Wednesday, the State Department said it had sent an unspecified number of US personnel stationed in China back to the United States for further medical testing.
In the alert, the State Department compared the employee's injuries to those suffered by US personnel and their families in Cuba between 2016 and 2017.
Those affected in Cuba were diagnosed a range of symptoms after being exposed to what officials have described as "acoustic attacks."
"If you or members of your family experience any unusual, unexplained physical symptoms of events, auditory or sensory phenomena ... please contact your health care provider," the alert said.
While US authorities have only confirmed that one staffer has been affected so far, State Department spokeswoman Heather Nauert said on Wednesday they had commissioned optional health screenings for all Guangzhou personnel.
"As a result of the screening process so far, the Department has sent a number of individuals for further evaluation and a comprehensive assessment of their symptoms and findings in the United States," she said in a statement.
Chinese Foreign Ministry spokeswoman Hua Chunying said in her daily press briefing on Thursday they had not been informed of any additional cases after the first individual but the Chinese government would start an investigation if the US requested.
"(So) far, we have found no reason or clue that would lead to the situation reported by the US side. China will ensure the safety of foreign diplomatic staff in China, including those from the US," she said.
There is still no explanation for the mysterious illnesses which affected a number of US personnel in Cuba more than a year ago.
The US citizens -- 24 in total -- suffered a variety of symptoms including sharp ear pain, headaches, ringing in one ear, vertigo, disorientation, attention issues and signs consistent with mild traumatic brain injury or concussion.
In nearly all cases the ailments were preceded by some sort of "acoustic element," such as a "high-pitched beam of sound" or a "baffling sensation akin to driving with the windows partially open in a car."
A study earlier this year outlined the extensiveness of the problem, but the State Department has not pointed to a specific cause behind the mysterious incidents.
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