BBC
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 person experienced “subtle and vague, but abnormal, sensations of sound and pressure”, it said.
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.
An embassy spokesperson told US media the employee suffered a mild traumatic brain injury while working at the US consulate in the city of Guangzhou.
“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,” read a statement from the department.
“The US government is taking these reports seriously and has informed its official staff in China of this event,” it said.
A warning on the website of the US Embassy and Consulate said: “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.”
“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 the condition of anonymity.
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 did not blame Cuba, and Cuba denied any involvement.