The US state department has urged its staff in China to alert them to any abnormal hearing or vision issues after one employee reported mystery symptoms.
The US previously said its diplomats in Cuba had suffered similar symptoms after suspected sonic attacks.
China-US relations have been strained recently, with the threat of a trade war looming.
The US has not accused China of being behind the issues.
What happened in China?
Embassy spokeswoman Jinnie Lee said the employee suffered a “variety of physical symptoms” between late 2017 and April 2018 while working at the US consulate in the city of Guangzhou.
The employee was sent back to the US, and on 18 May the embassy learnt that they had been diagnosed with mild traumatic brain injury (MTBI), Ms Lee added.
“We do not currently know what caused the reported symptoms and we are not aware of any similar situations in China, either inside or outside of the diplomatic community,” the US diplomatic statement said.
“The US government is taking these reports seriously and has informed its official staff in China of this event,” it said.
The statement continues with a warning: “While in China, if you experience any unusual acute auditory or sensory phenomena accompanied by unusual sounds or piercing noises, do not attempt to locate their source. Instead, move to a location where the sounds are not present.”
Ms Lee said the Chinese government had given assurances that it was also investigating and taking appropriate measures.
Was this a sonic attack?
Parallels have been drawn with the suspected sonic attacks in Havana.
However, there is nothing so far to link the occurrence with those in Cuba.
“We cannot at this time connect it with what happened in Havana but we are investigating all possibilities,” a US embassy official in Beijing told AFP news agency on condition of anonymity.
What happened in Cuba?
Around November 2016, US diplomats based in Cuba started to complain of odd ailments, including dizziness, nausea and hearing problems.
More than 20 members of staff in Havana were harmed in the “health attacks”, according to the state department. At least two Canadians were also affected.
The US has held Cuba responsible, either for allowing the suspected attacks to happen or for carrying them out itself.
Cuba has denied any involvement, and described the reports as a “political manipulation” aimed at damaging bilateral relations.