Anyone experiencing "any unusual, unexplained physical symptoms or events, auditory or sensory phenomena, or other health concerns" should seek medical advice, the alert said.
It also urged Americans to be on guard against symptoms including "dizziness, headaches, tinnitus, fatigue, cognitive issues, visual problems, ear complaints and hearing loss, and difficulty sleeping."
The mystery illness, apparently caused by unexplained sounds heard in diplomats' homes in China, appears similar to a series of cases that struck U.S. and Canadian diplomats and their families in Cuba beginning in late 2016. It has baffled medical experts and the FBI, despite extensive investigations and research.
The State Department said one employee from the consulate in the southern city of Guangzhou, who has already been evacuated from China, was found to have exhibited symptoms "consistent" with the Cuba incident, after hearing strange noises in his apartment and showing signs of brain injury.
Since that case was made public last month, an undisclosed number of other consular staff and family members have also been evacuated for further tests.
A U.S. medical team is in Guangzhou doing initial evaluations to see if more people need to be sent home, the Associated Press has reported.
The alert, the second in two weeks on the subject, told Americans they should not attempt to locate the source of any unidentified auditory sensation. Instead, it said, they should move to a different location.
In Cuba, 24 U.S. Embassy employees and family members were ultimately evacuated, along with 10 Canadians. The U.S. diplomatic presence in Havana was scaled back, and 15 Cuban diplomats were sent home from the United States in retaliation for their country's apparent failure to protect diplomats.
But an FBI investigation that lasted more than a year failed to find the cause of the sounds or the illness. The fact that the same phenomenon may now be happening in China raises a number of questions.
Was Cuba unfairly blamed for the first incident, and could a different country or actor be involved? Why have only Americans and Canadians so far been affected? Is there some other technical explanation that does not involve deliberate foul play?
China's government said it conducted an investigation after the first incident here but found "no cause or clue" as to what might be behind the symptoms.
The idea that this could be some sort of mass hysteria appeared to have been ruled out in a study by University of Pennsylvania researchers. It showed that patients suffered sustained injury to their brain networks without any history of head trauma, similar to a concussion but without a blow to the head.
No toxins have been found. A virus was thought unlikely because the patients showed no signs of fever, and no Cuban staff at the embassies fell ill.
Instead, most patients associated their symptoms with loud, painful noises heard inside their apartments, chirps, hums, tones and even sounds like metal scraping coming from a distinct direction. The noises were often associated with vibrations or air pressure changes, similar to air resonating in a moving car with the windows partially rolled down. As a result, they exhibited signs of damage to their hearing, vision, balance and memory.
Mark Lenzi, a Foreign Service officer who was evacuated from Guangzhou this week along with his wife and 3-year-old son, described the noise as something like "marbles bouncing and hitting a floor, then rolling on an incline with a static sound."
A group of researchers at the University of Michigan suggested that ultrasound signals from two or more transmitters could have accidentally interfered with each other to produce an audible signal.
The ultrasound signals could have come from eavesdropping devices, from jammers meant to block eavesdropping, or even from ultrasound pest repellents, the researchers said, raising the possibility that whatever or whoever caused the noise may have had no intent to harm.